Methods for IP Address Encryption and Obfuscation
draft-denis-ipcrypt-14
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| Document | Type | Active Internet-Draft (individual) | |
|---|---|---|---|
| Authors | Christopher Barber , Frank Denis | ||
| Last updated | 2026-08-24 | ||
| RFC stream | (None) | ||
| Intended RFC status | Informational | ||
| Formats | |||
| Stream | Stream state | (No stream defined) | |
| Consensus boilerplate | Unknown | ||
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| IESG | IESG state | I-D Exists | |
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draft-denis-ipcrypt-14
Network Working Group C. Barber
Internet-Draft Trim
Intended status: Informational F. Denis
Expires: 25 February 2027 Fastly Inc.
24 August 2026
Methods for IP Address Encryption and Obfuscation
draft-denis-ipcrypt-14
Abstract
This document specifies secure, efficient methods for Internet
Protocol (IP) address encryption and obfuscation in privacy-
preserving storage, logging, and analytics. Unlike truncation, which
destroys data irreversibly, these methods are reversible with the
encryption key while providing strong privacy guarantees.
Four modes are defined: ipcrypt-deterministic (format-preserving, IP-
address output), ipcrypt-pfx (prefix-preserving, native address
size), ipcrypt-nd and ipcrypt-ndx (non-deterministic with random
tweaks). All support high-performance processing at network speeds
and produce interoperable results across implementations.
Discussion Venues
This note is to be removed before publishing as an RFC.
Source for this draft and an issue tracker can be found at
https://github.com/jedisct1/draft-denis-ipcrypt.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
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Internet-Drafts are draft documents valid for a maximum of six months
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on 25 February 2027.
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Copyright Notice
Copyright (c) 2026 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Use Cases and Motivations . . . . . . . . . . . . . . . . 4
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 6
3. IP Address Conversion . . . . . . . . . . . . . . . . . . . . 7
3.1. Converting to a 16-Byte Representation . . . . . . . . . 7
3.1.1. IPv6 Addresses . . . . . . . . . . . . . . . . . . . 7
3.1.2. IPv4 Addresses . . . . . . . . . . . . . . . . . . . 8
3.2. Converting from a 16-Byte Representation to an IP
Address . . . . . . . . . . . . . . . . . . . . . . . . . 8
4. Generic Constructions . . . . . . . . . . . . . . . . . . . . 8
5. Deterministic Encryption . . . . . . . . . . . . . . . . . . 9
5.1. ipcrypt-deterministic . . . . . . . . . . . . . . . . . . 10
5.2. Format Preservation and Limitations . . . . . . . . . . . 10
5.2.1. Network Hierarchy Preservation . . . . . . . . . . . 10
5.2.2. Format Preservation Without Prefix Preservation . . . 11
5.2.3. Preserving Metadata for Analytics . . . . . . . . . . 11
6. Prefix-Preserving Encryption . . . . . . . . . . . . . . . . 12
6.1. Prefix-Preserving Construction . . . . . . . . . . . . . 12
6.2. Concrete Instantiation: ipcrypt-pfx . . . . . . . . . . . 13
6.2.1. Encryption Process . . . . . . . . . . . . . . . . . 13
6.2.2. Key Requirements . . . . . . . . . . . . . . . . . . 14
6.2.3. Security Properties . . . . . . . . . . . . . . . . . 14
6.2.4. Implementation Considerations . . . . . . . . . . . . 15
7. Non-Deterministic Encryption . . . . . . . . . . . . . . . . 15
7.1. Encryption Process . . . . . . . . . . . . . . . . . . . 16
7.2. Decryption Process . . . . . . . . . . . . . . . . . . . 16
7.3. Output Format and Encoding . . . . . . . . . . . . . . . 16
7.4. Concrete Instantiations . . . . . . . . . . . . . . . . . 16
7.4.1. ipcrypt-nd (KIASU-BC) . . . . . . . . . . . . . . . . 17
7.4.2. ipcrypt-ndx (AES-XTS) . . . . . . . . . . . . . . . . 17
7.4.3. Comparison of Modes . . . . . . . . . . . . . . . . . 18
7.5. Alternatives to Random Tweaks . . . . . . . . . . . . . . 19
8. Security Considerations . . . . . . . . . . . . . . . . . . . 19
8.1. Deterministic Mode Security . . . . . . . . . . . . . . . 20
8.2. Non-Deterministic Mode Security . . . . . . . . . . . . . 20
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8.3. Implementation Security . . . . . . . . . . . . . . . . . 20
8.4. Key Derivation for Multiple Variants . . . . . . . . . . 20
8.5. Key Management Considerations . . . . . . . . . . . . . . 21
9. Implementation Details . . . . . . . . . . . . . . . . . . . 22
9.1. Visual Diagrams . . . . . . . . . . . . . . . . . . . . . 22
9.1.1. IPv4 Address Conversion Diagram . . . . . . . . . . . 22
9.1.2. Deterministic Encryption Flow . . . . . . . . . . . . 22
9.1.3. Prefix-Preserving Encryption Flow (ipcrypt-pfx) . . . 22
9.1.4. Non-Deterministic Encryption Flow (ipcrypt-nd) . . . 23
9.1.5. Non-Deterministic Encryption Flow (ipcrypt-ndx) . . . 23
9.2. IPv4 Address Conversion . . . . . . . . . . . . . . . . . 24
9.3. IPv6 Address Conversion . . . . . . . . . . . . . . . . . 24
9.4. Conversion from a 16-Byte Array to an IP Address
String . . . . . . . . . . . . . . . . . . . . . . . . . 25
9.5. Deterministic Encryption (ipcrypt-deterministic) . . . . 26
9.5.1. Encryption . . . . . . . . . . . . . . . . . . . . . 26
9.5.2. Decryption . . . . . . . . . . . . . . . . . . . . . 27
9.6. Prefix-Preserving Encryption (ipcrypt-pfx) . . . . . . . 27
9.6.1. Encryption . . . . . . . . . . . . . . . . . . . . . 27
9.6.2. Decryption . . . . . . . . . . . . . . . . . . . . . 28
9.6.3. Helper Functions . . . . . . . . . . . . . . . . . . 30
9.7. Non-Deterministic Encryption using KIASU-BC
(ipcrypt-nd) . . . . . . . . . . . . . . . . . . . . . . 31
9.7.1. Encryption . . . . . . . . . . . . . . . . . . . . . 31
9.7.2. Decryption . . . . . . . . . . . . . . . . . . . . . 32
9.8. Non-Deterministic Encryption using AES-XTS
(ipcrypt-ndx) . . . . . . . . . . . . . . . . . . . . . . 32
9.8.1. Encryption . . . . . . . . . . . . . . . . . . . . . 32
9.8.2. Decryption . . . . . . . . . . . . . . . . . . . . . 33
9.8.3. Helper Functions for AES-XTS . . . . . . . . . . . . 33
9.9. KIASU-BC Implementation Guide . . . . . . . . . . . . . . 34
9.9.1. Overview . . . . . . . . . . . . . . . . . . . . . . 34
9.9.2. Tweak Padding . . . . . . . . . . . . . . . . . . . . 34
9.9.3. Round Structure . . . . . . . . . . . . . . . . . . . 35
9.9.4. Key Schedule . . . . . . . . . . . . . . . . . . . . 35
9.9.5. Implementation Steps . . . . . . . . . . . . . . . . 35
9.9.6. Example Implementation . . . . . . . . . . . . . . . 36
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 38
10.1. Normative References . . . . . . . . . . . . . . . . . . 38
10.2. Informative References . . . . . . . . . . . . . . . . . 39
Appendix A. Test Vectors . . . . . . . . . . . . . . . . . . . . 40
A.1. ipcrypt-deterministic Test Vectors . . . . . . . . . . . 40
A.2. ipcrypt-pfx Test Vectors . . . . . . . . . . . . . . . . 41
A.2.1. Basic Test Vectors . . . . . . . . . . . . . . . . . 41
A.2.2. Prefix-Preserving Test Vectors . . . . . . . . . . . 42
A.3. ipcrypt-nd Test Vectors . . . . . . . . . . . . . . . . . 43
A.4. ipcrypt-ndx Test Vectors . . . . . . . . . . . . . . . . 43
IANA Considerations . . . . . . . . . . . . . . . . . . . . . . . 44
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Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 44
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 44
1. Introduction
Internet Protocol (IP) addresses are personally identifiable
information requiring protection, yet common anonymization approaches
have fundamental limitations. Truncation (zeroing parts of
addresses) irreversibly destroys data while providing variable
privacy levels; a /24 mask may obscure one user or thousands
depending on network allocation. Hashing produces non-reversible
outputs unsuitable for operational tasks such as abuse investigation.
Ad-hoc encryption schemes often lack rigorous security analysis and
have limited interoperability.
This document addresses these deficiencies by specifying secure,
efficient, and interoperable methods for IP address encryption and
obfuscation.
This specification addresses concerns raised in [RFC7624] regarding
confidentiality when sharing data with third parties. Unlike
existing practices that obscure addresses, these methods provide
well-defined security properties, which are discussed throughout this
document and summarized in Section 8.
1.1. Use Cases and Motivations
Organizations handling IP addresses require mechanisms to protect
user privacy while maintaining operational capabilities. Generic
encryption systems expand data unpredictably, lack compatibility with
network tools, and are not designed for high-volume processing. The
specialized methods in this specification address these requirements:
* Efficiency and Compactness: All variants operate on 128 bits,
achieving single-block encryption speed required for network-rate
processing. Non-deterministic variants add only 8-16 bytes of
tweak overhead.
* High Usage Limits: Non-deterministic variants safely handle
massive volumes: approximately 4 billion operations for ipcrypt-nd
and 18 quintillion for ipcrypt-ndx per key, without degrading
security. Generic encryption often requires complex key rotation
schemes at lower thresholds.
* Format Preservation: The ipcrypt-deterministic and ipcrypt-pfx
variants produce valid IP addresses rather than arbitrary
ciphertext, enabling encrypted addresses to pass through existing
network infrastructure, monitoring tools, and databases without
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modification. The ipcrypt-pfx variant uniquely preserves network
prefix relationships while maintaining the original address type
and size, enabling network-level analytics while protecting
individual address identity (see Section 5.2).
* Interoperability: All conforming implementations produce identical
results, enabling data exchange between different systems,
vendors, and programming languages.
These specialized encryption methods enable several use cases:
* Privacy Protection: Prevents exposure of sensitive user
information to third parties in logs, analytics data, and network
measurements ([RFC6973]). The key holder retains decryption
capability.
* Correlation Attack Resistance: Non-deterministic variants leverage
random tweaks to hide patterns and enhance confidentiality (see
Section 7).
* Privacy-Preserving Analytics: Encrypted IP addresses can be used
directly for counting unique clients, rate limiting, or
deduplication without revealing original values. This addresses
the anonymization requirements for DNS query data sharing outlined
in [RSSAC040]. The ipcrypt-pfx variant preserves network prefixes
for network-level analytics, while other methods scramble network
hierarchy.
* Third-Party Integration: Encrypted IP addresses can serve as
privacy-preserving identifiers when interacting with untrusted
services, cloud providers, or external platforms.
The following examples demonstrate how the same IP addresses
transform under each method:
* Format-preserving: Valid IP addresses, same input always produces
same output:
192.168.1.1 -> d1e9:518:d5bc:4487:51c6:c51f:44ed:e9f6
192.168.1.254 -> fd7e:f70f:44d7:cdb2:2992:95a1:e692:7696
192.168.1.254 -> fd7e:f70f:44d7:cdb2:2992:95a1:e692:7696 # Repeated
* Prefix-preserving: Maintains network structure, same prefix when
IPs share prefix:
192.168.1.1 -> 251.81.131.124
192.168.1.254 -> 251.81.131.159 # Prefix is preserved
172.16.69.42 -> 165.228.146.177
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* Non-deterministic: Larger output, different each time:
192.168.1.1 -> f0ea0bbd...03aa9fcb
192.168.1.254 -> 620b58d8...2ff8086f
192.168.1.254 -> 35fc2338...25abed5d # Same input, different outputs
These methods achieve optimal efficiency for their security goals:
each variant uses the minimum ciphertext expansion and fewest
cryptographic operations needed for its properties (format
preservation, prefix preservation, or correlation resistance).
[IPCRYPT].
For implementation guidelines, see Section 9.
2. Terminology
The key words “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”,
“SHOULD”, “SHOULD NOT”, “RECOMMENDED”, “NOT RECOMMENDED”, “MAY”, and
“OPTIONAL” in this document are to be interpreted as described in BCP
14 [RFC8174] when, and only when, they appear in all capitals, as
shown here.
Throughout this document, the following terms and conventions apply:
* IP Address: An IPv4 or IPv6 address as defined in [RFC4291].
* IPv4-mapped IPv6 Address: An IPv6 address format (::FFFF:a.b.c.d)
used to represent IPv4 addresses within the IPv6 address space,
enabling uniform processing of both address types.
* 16-Byte Representation: A fixed-length representation used for
both IPv4 (via IPv4-mapped IPv6) and IPv6 addresses.
* Block Cipher: A deterministic cryptographic algorithm that
encrypts fixed-size blocks of data (128 bits with AES) using a
secret key.
* Permutation: A bijective function where each distinct input maps
to a unique output, ensuring reversibility.
* Pseudorandom Function (PRF): A deterministic function that
produces output computationally indistinguishable from random
values.
* Tweakable Block Cipher (TBC): A block cipher that accepts an
additional non-secret parameter (tweak) along with the key and
plaintext, allowing domain separation without changing keys
[LRW2002].
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* Exclusive OR (XOR): A bitwise operation whose output is 1 when its
two input bits differ and 0 otherwise.
* Tweak: A non-secret, additional input to a tweakable block cipher
that further randomizes the output.
* Deterministic Encryption: Encryption that always produces the same
ciphertext for a given input and key.
* Non-Deterministic Encryption: Encryption that produces different
ciphertexts for the same input due to the inclusion of a randomly
sampled tweak.
* Prefix-Preserving Encryption: An encryption mode where IP
addresses from the same network produce ciphertexts that share a
common encrypted prefix, maintaining network relationships while
obscuring actual network identities.
* Birthday Bound: The point at which collisions become statistically
likely in a random sampling process, approximately 2^(n/2)
operations for n-bit values.
* (Input, Tweak) Collision: A scenario where the same input is
encrypted with the same tweak. This reveals that the input was
repeated but not the input’s value.
3. IP Address Conversion
This section describes the conversion of IP addresses to and from a
16-byte representation used by ipcrypt-deterministic, ipcrypt-nd, and
ipcrypt-ndx. The ipcrypt-pfx method differs by maintaining native
address sizes—4 bytes for IPv4 and 16 bytes for IPv6—to preserve
network structure (see Section 6).
3.1. Converting to a 16-Byte Representation
3.1.1. IPv6 Addresses
IPv6 addresses (128 bits) are converted directly using network byte
order as specified in [RFC4291].
_Example:_
IPv6 Address: 2001:0db8:85a3:0000:0000:8a2e:0370:7334
16-Byte Representation:
[20 01 0d b8 85 a3 00 00 00 00 8a 2e 03 70 73 34]
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3.1.2. IPv4 Addresses
IPv4 addresses (32 bits) are mapped using the IPv4-mapped IPv6 format
as specified in [RFC4291]:
IPv4 Address: 192.0.2.1
16-Byte Representation:
[00 00 00 00 00 00 00 00 00 00 FF FF C0 00 02 01]
Note on representation equivalence: For all ipcrypt variants,
encrypting an IPv4 address and encrypting its IPv4-mapped IPv6 form
(::FFFF:a.b.c.d) produce the same encrypted output.
This behavior is intentional: in the intended use cases, IP addresses
function as identifiers, and producing a single encrypted identifier
for the same endpoint across representations avoids ambiguity.
3.2. Converting from a 16-Byte Representation to an IP Address
Conversion algorithm:
1. Examine the first 12 bytes of the 16-byte representation
2. If they match the IPv4-mapped prefix (10 bytes of 0x00 followed
by 0xFF, 0xFF):
* Interpret the last 4 bytes as an IPv4 address in dotted-
decimal notation
3. Otherwise:
* Interpret the 16 bytes as an IPv6 address in colon-hexadecimal
notation
4. Generic Constructions
This specification defines two generic cryptographic constructions:
1. 128-bit Block Cipher Construction:
* Used in deterministic encryption (see Section 5)
* Operates on a single 16-byte block
* Example: AES-128 treated as a permutation
2. 128-bit Tweakable Block Cipher (TBC) Construction:
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* Used in non-deterministic encryption (see Section 7)
* Accepts a key, a tweak, and a message
* The tweak must be uniformly random when generated
* Reuse of the same tweak on different inputs does not
compromise confidentiality
Valid options for implementing a tweakable block cipher include, but
are not limited to:
* SKINNY (see [SKINNY])
* DEOXYS-TBC (see [DEOXYS-TBC])
* KIASU-BC (see Section 9.9 for implementation details)
* AES-XTS (see Section 7.4.2 for usage)
Implementers MUST choose a cipher that provides robust resistance
against related-tweak and other cryptographic attacks.
5. Deterministic Encryption
Deterministic encryption applies a 128-bit block cipher directly to
the 16-byte representation of an IP address, producing the same
ciphertext for the same input and key.
Deterministic encryption is appropriate when:
* Duplicate IP addresses need to be detected in encrypted form
(e.g., for rate limiting)
* Storage space is critical (produces only 16 bytes output)
* Format preservation is required (output remains a valid IP
address)
* Correlation of the same address across records is acceptable
All instantiations (ipcrypt-deterministic, ipcrypt-pfx, ipcrypt-nd,
and ipcrypt-ndx) are invertible. For non-deterministic modes, the
tweak must be preserved along with the ciphertext to enable
decryption.
Implementation details are provided in Section 9.
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5.1. ipcrypt-deterministic
The ipcrypt-deterministic instantiation employs AES-128 in a single-
block operation. The key MUST be 16 bytes (128 bits). As AES-128 is
a permutation, each distinct input maps to a unique ciphertext,
producing a valid IP address representation.
Test vectors are provided in Appendix A.1.
+---------------------+
| IP Address |
| (IPv4 or IPv6) |
+---------------------+
|
v
+---------------------+
| Convert to 16 Bytes |
+---------------------+
|
v
+---------------------+
| AES-128 Encrypt |
| (Single Block) |
+---------------------+
|
v
+---------------------+
| 16-Byte Output |
+---------------------+
|
v
+---------------------+
| Convert to IP Format|
+---------------------+
5.2. Format Preservation and Limitations
5.2.1. Network Hierarchy Preservation
Most encryption methods in this specification scramble network
hierarchy, with the notable exception of ipcrypt-pfx:
* ipcrypt-deterministic, ipcrypt-nd, and ipcrypt-ndx: These methods
completely scramble IPv4 and IPv6 prefixes in the encrypted
output. Addresses from the same subnet appear unrelated after
encryption, and geographic or topological proximity cannot be
inferred.
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* ipcrypt-pfx: This method preserves network prefix relationships in
the encrypted output. Addresses from the same subnet share a
common encrypted prefix, enabling network-level analytics while
protecting the actual network identity. The encrypted prefixes
themselves are cryptographically transformed and unrecognizable
without the key.
5.2.2. Format Preservation Without Prefix Preservation
For methods other than ipcrypt-pfx, IPv4 addresses are typically
encrypted as IPv6 addresses.
IPv4 format preservation without prefix preservation (maintaining
IPv4 addresses as IPv4 in deterministic or non-deterministic modes)
is not specified in this document and is generally discouraged due to
the limited 32-bit address space, which significantly reduces
encryption security.
If IPv4 format preservation is absolutely required despite the
security limitations, implementers SHOULD implement a format-
preserving encryption (FPE) mode [BRW2005], such as the FF1 algorithm
specified in [NIST-SP-800-38G] or FAST [FAST].
5.2.3. Preserving Metadata for Analytics
Organizations requiring network metadata for analytics have two
options:
1. Use ipcrypt-pfx to preserve network structure within the
encrypted addresses, enabling network-level analysis while
keeping actual network identities encrypted.
2. For non-prefix-preserving modes (ipcrypt-deterministic, ipcrypt-
nd, ipcrypt-ndx), extract and store metadata (geographic
location, autonomous system number (ASN), network classification)
as separate fields before encryption.
Both approaches provide advantages over IP address truncation, which
provides inconsistent protection and irreversibly destroys data.
When auxiliary information such as AS number and geographical
location is required, it SHOULD be stored as metadata at the time of
logging. Performing these mappings later may yield different results
as network allocations and routing information change over time.
This recommendation applies to all encryption modes: even with
ipcrypt-pfx, the preserved network structure does not retain these
additional attributes.
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6. Prefix-Preserving Encryption
Prefix-preserving encryption maintains network structure in encrypted
IP addresses. Addresses from the same network produce encrypted
addresses that share a common prefix, enabling privacy-preserving
network analytics while preventing identification of specific
networks or users.
Unlike standard encryption that completely scrambles addresses,
prefix-preserving encryption enables network operators to:
* Detect traffic patterns from common networks without knowing which
specific networks
* Perform network-level rate limiting on encrypted addresses
* Implement distributed denial-of-service (DDoS) mitigation while
preserving user privacy
* Analyze network topology without accessing raw IP addresses
This mode balances privacy with analytical capability: individual
addresses remain encrypted and network identities are
cryptographically transformed, but network relationships remain
visible through shared encrypted prefixes.
6.1. Prefix-Preserving Construction
The encryption process achieves prefix preservation through a bit-by-
bit transformation that maintains consistency across addresses with
shared prefixes. For any two IP addresses sharing the first N bits,
their encrypted forms also share the first N bits. This property
holds recursively for all prefix lengths.
The algorithm operates as follows:
1. Process each bit position sequentially from most significant to
least significant
2. For each bit position, extract the prefix (all bits processed so
far) from the original IP address
3. Apply a pseudorandom function (PRF) that takes the padded prefix
as input to generate a cipher bit
4. XOR the cipher bit with the original bit at the current position
to produce the encrypted bit
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5. The encrypted bit depends deterministically on the prefix from
the original IP, ensuring identical prefixes always produce
identical encrypted prefixes
This construction ensures:
* Identical prefixes always produce identical transformations for
subsequent bits
* Different prefixes produce cryptographically distinct
transformations
* The transformation is deterministic yet cryptographically secure
* Network relationships are preserved while actual network
identities remain encrypted
The algorithm maintains native address sizes: IPv4 addresses remain 4
bytes (32 bits) and IPv6 addresses remain 16 bytes (128 bits).
6.2. Concrete Instantiation: ipcrypt-pfx
The ipcrypt-pfx instantiation implements prefix-preserving encryption
using a pseudorandom function based on the XOR of two independently
keyed AES-128 encryptions.
6.2.1. Encryption Process
The encryption uses a pseudorandom function based on the XOR of two
independently keyed AES-128 encryptions. The 32-byte key is split
into two independent 16-byte AES-128 keys (K1 and K2).
For each bit position (processing from the most significant bit (MSB)
to the least significant bit (LSB)):
1. Prefix Padding: The prefix (all bits processed so far from the
original IP address) is padded to 128 bits using the format
zeros || 1 || prefix_bits, where:
* The prefix bits are extracted from the most significant bits
of the original IP address
* A single 1 bit serves as a delimiter at position
prefix_len_bits
* The prefix bits are placed immediately after the delimiter,
from high to low positions
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* For an empty prefix (processing the first bit), this produces
a block with only a single 1 bit at position 0
2. Pseudorandom Function Computation: The padded prefix is encrypted
independently with both K1 and K2, producing two 128-bit outputs
(e1 and e2). The final PRF output is computed as e = e1 ⊕ e2.
3. Bit Encryption: The least significant bit is extracted from the
PRF output as the cipher bit, which is then XORed with the
original bit at the current position to produce the encrypted
bit.
Complete pseudocode implementation is provided in Section 9.6.
6.2.2. Key Requirements
The two 16-byte halves of the 32-byte key (K1 and K2) MUST NOT be
identical. Using identical values for K1 and K2 (e.g., repeating the
same 16 bytes twice) causes the XOR operation to cancel out,
returning the original IP address unchanged.
When the 32-byte key is randomly sampled from a uniform distribution,
the probability that K1 = K2 is statistically negligible.
6.2.3. Security Properties
The ipcrypt-pfx construction improves upon earlier designs such as
CRYPTO-Pan through enhanced cryptographic security:
* Sum-of-Permutations: The XOR of two independently keyed AES-128
permutations provides 128-bit PRF security
[SUM-OF-PRPS][PATARIN-2008][PATARIN-H-COEFFICIENTS], with
distinguishing advantage growing on the order of q/2^128 for q
queries. This construction ensures robust security even for
massive-scale deployments.
* Prefix-based context isolation: shift-and-append updates make each
bit depend on the full prefix history and ensure fresh PRF input
each round.
* Fixed network-order representation and explicit bit indexing avoid
endianness pitfalls.
Note: Prefix-preserving encryption intentionally reveals network
structure to enable analytics. Organizations requiring complete
address obfuscation should use non-prefix-preserving modes.
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6.2.4. Implementation Considerations
Key implementation characteristics:
* Computational Requirements:
- IPv4: 64 AES-128 operations per address (2 encryptions × 32
bits)
- IPv6: 256 AES-128 operations per address (2 encryptions × 128
bits)
* Performance Optimizations:
- Caching encrypted prefix values (e1 and e2) significantly
improves performance for addresses sharing common prefixes
- The encryption algorithm is inherently parallelizable since AES
computations for different bit positions are independent
- The padded prefix computation can be optimized by maintaining
state across iterations: instead of recomputing the padded
prefix from scratch for each bit position, implementations can
shift the previous padded prefix left by one bit and insert the
next input bit.
7. Non-Deterministic Encryption
Non-deterministic encryption enhances privacy by ensuring that the
same IP address produces different ciphertexts each time it is
encrypted, preventing correlation attacks that plague deterministic
schemes. This is achieved through tweakable block ciphers that
incorporate random values called tweaks.
Non-deterministic encryption is appropriate when:
* Preventing correlation of the same IP address across records is
critical
* Storage can accommodate the additional tweak data (8-16 bytes)
* Stronger privacy guarantees than deterministic encryption provides
are required
* Processing the same address multiple times without revealing
repetition patterns
Implementation details are provided in Section 9.
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7.1. Encryption Process
Encryption process for non-deterministic modes:
1. Generate a random tweak using a cryptographically secure random
number generator
2. Convert the IP address to its 16-byte representation
3. Encrypt the 16-byte representation using the key and the tweak
4. Concatenate the tweak with the encrypted output to form the final
ciphertext
The tweak is not considered secret and is included in the ciphertext,
enabling its use for decryption.
7.2. Decryption Process
Decryption process:
1. Split the ciphertext into the tweak and the encrypted IP
2. Decrypt the encrypted IP using the key and the tweak
3. Convert the resulting 16-byte representation back to an IP
address
Although the tweak is generated uniformly at random, occasional
collisions may occur according to birthday bounds. An (input, tweak)
collision reveals that the same input was encrypted with the same
tweak but does not disclose the input’s value. The usage limits
apply per cryptographic key; rotating keys extends secure usage
beyond these bounds.
7.3. Output Format and Encoding
The output of non-deterministic encryption is binary data. For text
representation, the binary output MUST be encoded (e.g., hexadecimal
or Base64). Implementations SHOULD document their chosen encoding
method.
7.4. Concrete Instantiations
This document defines two concrete instantiations:
* ipcrypt-nd: Uses the KIASU-BC tweakable block cipher with an
8-byte (64-bit) tweak. See [KIASU-BC] for details.
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* ipcrypt-ndx: Uses the AES-XTS tweakable block cipher with a
16-byte (128-bit) tweak. See [IEEE-P1619] for background.
In both cases, if a tweak is generated randomly, it MUST be uniformly
random. Reusing the same randomly generated tweak on different
inputs is acceptable from a confidentiality standpoint.
Test vectors are provided in Appendix A.3 and Appendix A.4.
7.4.1. ipcrypt-nd (KIASU-BC)
The ipcrypt-nd instantiation uses the KIASU-BC tweakable block cipher
with an 8-byte (64-bit) tweak. Implementation details are provided
in Section 9.9. The output is 24 bytes total, consisting of an
8-byte tweak concatenated with a 16-byte ciphertext.
Random sampling of an 8-byte tweak yields an expected collision after
about 2^32 operations (approximately 4 billion). An (input, tweak)
collision indicates repetition without revealing the input’s value.
These collision bounds apply per cryptographic key. Regular key
rotation can extend secure usage beyond these bounds. The effective
security is determined by the underlying block cipher’s strength.
Test vectors are provided in Appendix A.3.
7.4.2. ipcrypt-ndx (AES-XTS)
The ipcrypt-ndx instantiation uses the AES-XTS tweakable block cipher
with a 16-byte (128-bit) tweak. The output is 32 bytes total,
consisting of a 16-byte tweak concatenated with a 16-byte ciphertext.
Since only a single block is encrypted, the construction is
equivalent to AES-XEX, and identical to AES-XTS at block index 0,
where the tweak is not multiplied by the primitive element α.
For single-block AES-XTS, independent sampling of a 16-byte tweak
results in an expected collision after about 2^64 operations
(approximately 18 quintillion).
Similar to ipcrypt-nd, collisions reveal repetition without
compromising the input value. These limits are per key, and regular
key rotation extends secure usage. The effective security is
governed by AES-128 strength (approximately 2^128 operations).
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7.4.3. Comparison of Modes
Mode selection depends on specific privacy requirements and
operational constraints:
* Deterministic (ipcrypt-deterministic):
- Output size: 16 bytes (most compact)
- Privacy: Same IP always produces same ciphertext (allows
correlation)
- Use case: When duplicate identification is needed or when
format preservation is critical
- Performance: Fastest (single AES operation)
* Prefix-Preserving (ipcrypt-pfx):
- Output size: 4 bytes for IPv4, 16 bytes for IPv6 (maintains
native sizes)
- Privacy: Preserves network prefix relationships while
encrypting actual network identities
- Use case: Network analytics, traffic pattern analysis, subnet
monitoring, DDoS mitigation
- Performance: Bit-by-bit processing (64 AES operations for IPv4,
256 for IPv6), fully parallelizable
* Non-Deterministic ipcrypt-nd (KIASU-BC):
- Output size: 24 bytes (16-byte ciphertext + 8-byte tweak)
- Privacy: Same IP produces different ciphertexts (prevents most
correlation)
- Use case: General privacy protection with reasonable storage
overhead
- Collision resistance: Approximately 4 billion operations per
key
* Non-Deterministic ipcrypt-ndx (AES-XTS):
- Output size: 32 bytes (16-byte ciphertext + 16-byte tweak)
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- Privacy: Same IP produces different ciphertexts (prevents
correlation)
- Use case: Maximum privacy protection when storage permits
- Collision resistance: Approximately 18 quintillion operations
per key
7.5. Alternatives to Random Tweaks
While this specification recommends uniformly random tweaks for non-
deterministic encryption, alternative approaches may be considered:
* Monotonic Counter: A counter could be used as a tweak, but this is
difficult to maintain in distributed systems. If the counter is
not encrypted and the tweakable block cipher is not secure against
related-tweak attacks, this could enable correlation attacks.
* Universally unique identifiers (UUIDs), such as UUIDv6 or UUIDv7,
could be used as tweaks; however, these would reveal the original
timestamp of the logged IP addresses, which may not be desirable
from a privacy perspective.
Although the birthday bound presents considerations with random
tweaks, random tweaks remain the recommended approach for practical
deployments.
8. Security Considerations
The methods specified in this document provide strong confidentiality
guarantees but explicitly do not provide integrity protection:
These methods provide protection against:
* Unauthorized parties learning the original IP addresses (without
the key)
* Statistical analysis revealing patterns in network traffic (non-
deterministic modes)
* Brute-force attacks on the address space (128-bit security level)
These methods do not provide protection against:
* Active attackers modifying, reordering, or removing encrypted
addresses
* Authorized key holders decrypting addresses (by design)
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* Traffic analysis based on volume and timing (metadata)
Applications requiring integrity protection must additionally employ
authentication mechanisms such as HMAC, authenticated encryption
modes, or digital signatures over the encrypted data.
8.1. Deterministic Mode Security
A permutation ensures distinct inputs yield distinct outputs.
Repeated inputs result in identical ciphertexts, revealing
repetition.
This makes deterministic encryption suitable for applications where
format preservation is required and linkability is acceptable.
8.2. Non-Deterministic Mode Security
The inclusion of a random tweak ensures that encrypting the same
input generally produces different outputs. An (input, tweak)
collision reveals only that the same input was processed with that
tweak, not the input’s value.
Security is determined by the underlying block cipher (≈2^128 for
AES-128) on a per-key basis. Key rotation is recommended to extend
secure usage beyond the per-key collision bounds.
8.3. Implementation Security
Implementations MUST ensure that:
1. Keys are generated using a cryptographically secure random number
generator
2. Tweak values are uniformly random for non-deterministic modes
3. Side-channel attacks are mitigated through constant-time
operations
4. Error handling does not leak sensitive information
8.4. Key Derivation for Multiple Variants
When using multiple encryption variants within the same deployment,
implementations MUST derive separate keys for each variant to prevent
cross-mode correlations. The RECOMMENDED approach uses the HMAC-
based key derivation function (HKDF) [RFC5869] to derive per-variant
subkeys from a single root key. The pseudorandom key (PRK) is used
as follows:
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* K_deterministic = HKDF-Expand(PRK, "ipcrypt-deterministic", 16)
* K_pfx = HKDF-Expand(PRK, "ipcrypt-pfx", 32)
* K_nd = HKDF-Expand(PRK, "ipcrypt-nd", 16)
* K_ndx = HKDF-Expand(PRK, "ipcrypt-ndx", 32)
Where:
* PRK = HKDF-Extract(salt, K_root) is a pseudorandom key derived
from the root key
* K_root is a uniformly random root key
* salt is either empty or a fixed random value for the application
* The strings "ipcrypt-deterministic", etc. are used as the info
parameter for domain separation
* The third parameter specifies the output length in bytes (16 for
single AES keys, 32 for ipcrypt-pfx and ipcrypt-ndx)
This ensures that:
1. Using the same root key across different variants does not enable
cross-variant attacks
2. Key management is simplified by requiring only a single root key
3. Each variant operates with cryptographically independent keys
8.5. Key Management Considerations
Implementers MUST ensure:
1. Keys are generated using cryptographically secure random number
generators (see [RFC4086])
2. Keys are stored securely and access-controlled appropriately for
the deployment environment
3. Key rotation policies are established based on usage volume and
security requirements
4. Key compromise procedures are defined and tested
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9. Implementation Details
This section provides pseudocode and implementation guidance for the
operations described in this document.
In the pseudocode throughout this document, the notation “for i from
x to y” indicates iteration starting at x (inclusive) and ending
before y (exclusive). For example, “for i from 0 to 4” processes
values 0, 1, 2, and 3, but not 4.
9.1. Visual Diagrams
The following diagrams illustrate the processes described in this
specification.
9.1.1. IPv4 Address Conversion Diagram
IPv4: 192.0.2.1
|
v
Octets: C0 00 02 01
|
v
16-Byte Array:
[00 00 00 00 00 00 00 00 00 00 | FF FF | C0 00 02 01]
9.1.2. Deterministic Encryption Flow
IP Address
|
v
[Convert to 16 Bytes]
|
v
[AES-128 Single-Block Encrypt]
|
v
16-Byte Ciphertext
|
v
[Convert to IP Format]
|
v
Encrypted IP Address
9.1.3. Prefix-Preserving Encryption Flow (ipcrypt-pfx)
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IP Address
|
v
[Convert to 16 Bytes]
|
v
[Split 32-byte key into K1, K2]
|
v
[For each bit position (MSB to LSB):
- Pad current prefix to 128 bits:
zeros || 1 || prefix_bits
- e1 = AES(K1, padded_prefix)
- e2 = AES(K2, padded_prefix)
- e = e1 ⊕ e2
- Extract LSB from e as cipher_bit
- XOR cipher_bit with original bit
- Set result bit in encrypted output
- Add original bit to prefix for next iteration]
|
v
Encrypted IP Address
(4 bytes for IPv4, 16 bytes for IPv6)
9.1.4. Non-Deterministic Encryption Flow (ipcrypt-nd)
IP Address
|
v
[Convert to 16 Bytes]
|
v
[Generate Random 8-Byte Tweak]
|
v
[KIASU-BC Tweakable Encrypt]
|
v
16-Byte Ciphertext
|
v
[Concatenate Tweak || Ciphertext]
|
v
24-Byte Output (ipcrypt-nd)
9.1.5. Non-Deterministic Encryption Flow (ipcrypt-ndx)
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IP Address
|
v
[Convert to 16 Bytes]
|
v
[Generate Random 16-Byte Tweak]
|
v
[AES-XTS Tweakable Encrypt]
|
v
16-Byte Ciphertext
|
v
[Concatenate Tweak || Ciphertext]
|
v
32-Byte Output (ipcrypt-ndx)
9.2. IPv4 Address Conversion
See Section 9.1.1 for a diagram of this process.
function ipv4_to_16_bytes(ipv4_address):
// Parse the IPv4 address into 4 octets
octets = parseIPv4(ipv4_address) // Returns 4 octets
// Create a 16-byte array with the IPv4-mapped prefix
bytes16 = [0x00] * 10 // 10 bytes of 0x00
bytes16.append(0xFF) // 11th byte: 0xFF
bytes16.append(0xFF) // 12th byte: 0xFF
// Append each octet (converted to an 8-bit integer)
for octet in octets:
bytes16.append(octet)
return bytes16
_Example:_ For "192.0.2.1", the function returns
[00, 00, 00, 00, 00, 00, 00, 00, 00, 00, FF, FF, C0, 00, 02, 01]
9.3. IPv6 Address Conversion
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function ipv6_to_16_bytes(ipv6_address):
// Parse the IPv6 address into eight 16-bit words.
words = parseIPv6(ipv6_address)
// Expands shorthand notation and returns eight words
bytes16 = []
for word in words:
high_byte = (word >> 8) & 0xFF
low_byte = word & 0xFF
bytes16.append(high_byte)
bytes16.append(low_byte)
return bytes16
_Example:_ For "2001:0db8:85a3:0000:0000:8a2e:0370:7334", the output
is the corresponding 16-byte sequence {0x20, 0x01, 0x0d, 0xb8, ...,
0x34}.
9.4. Conversion from a 16-Byte Array to an IP Address String
This function converts a 16-byte array back to an IP address string.
For IPv6 addresses, the output conforms to [RFC5952]. Implementers
SHOULD use existing IP address formatting functions from standard
libraries.
function bytes_16_to_ip(bytes16):
if length(bytes16) != 16:
raise Error("Invalid byte array")
// Check for the IPv4-mapped prefix
// When an IPv4-mapped IPv6 address (::ffff:x.x.x.x) is
// detected, convert it back to IPv4 format. This is expected
// because IPv4 addresses are represented as IPv4-mapped IPv6
// addresses for uniform processing.
ipv4_mapped_prefix = [
0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
0x00, 0x00, 0x00, 0x00, 0xFF, 0xFF
]
if bytes16[0..12] == ipv4_mapped_prefix:
// Convert the 4 last bytes to an IPv4 address
ipv4_parts = []
for i from 12 to 16:
ipv4_parts.append(integer_to_string(bytes16[i]))
ipv4_address = join(ipv4_parts, ".")
return ipv4_address
else:
// Convert the 16 bytes to a canonical IPv6 address
words = []
for i from 0 to 8:
word = (bytes16[i*2] << 8) | bytes16[i*2+1]
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words.append(word)
// Find longest run of consecutive zeros for compression
best_run_start = -1
best_run_length = 0
run_start = -1
for i from 0 to 8:
if words[i] == 0:
if run_start == -1:
run_start = i
else:
if run_start != -1:
run_length = i - run_start
if run_length > best_run_length:
best_run_start = run_start
best_run_length = run_length
run_start = -1
// Check final run
if run_start != -1:
run_length = 8 - run_start
if run_length > best_run_length:
best_run_start = run_start
best_run_length = run_length
// Build IPv6 string with zero compression
parts = []
i = 0
while i < 8:
if best_run_length >= 2 and i == best_run_start:
// Insert :: for compressed zeros
parts.append("" if i == 0 else ":")
parts.append("")
i += best_run_length
else:
parts.append(format_hex(words[i]))
i += 1
return join(parts, ":")
9.5. Deterministic Encryption (ipcrypt-deterministic)
9.5.1. Encryption
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function ipcrypt_deterministic_encrypt(ip_address, key):
// The key MUST be exactly 16 bytes (128 bits) in length
if length(key) != 16:
raise Error("Key must be 16 bytes")
bytes16 = convert_to_16_bytes(ip_address)
ciphertext = AES128_encrypt(key, bytes16)
encrypted_ip = bytes_16_to_ip(ciphertext)
return encrypted_ip
9.5.2. Decryption
function ipcrypt_deterministic_decrypt(encrypted_ip, key):
if length(key) != 16:
raise Error("Key must be 16 bytes")
bytes16 = convert_to_16_bytes(encrypted_ip)
plaintext = AES128_decrypt(key, bytes16)
original_ip = bytes_16_to_ip(plaintext)
return original_ip
9.6. Prefix-Preserving Encryption (ipcrypt-pfx)
9.6.1. Encryption
function ipcrypt_pfx_encrypt(ip_address, key):
// The key MUST be exactly 32 bytes (256 bits)
if length(key) != 32:
raise Error("Key must be 32 bytes")
// Convert IP to 16-byte representation
bytes16 = convert_to_16_bytes(ip_address)
// Split the key into two AES-128 keys
// IMPORTANT: K1 and K2 MUST be different
K1 = key[0:16]
K2 = key[16:32]
// Initialize encrypted result with zeros
encrypted = [0] * 16
// For IPv4, start at bit 96, where the address starts.
// The first 12 bytes already contain the IPv4-mapped prefix.
// This provides domain separation from the first 32 bits of
// an IPv6 address.
if is_ipv4_mapped(bytes16):
prefix_start = 96
// Set up the IPv4-mapped IPv6 prefix
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encrypted[10] = 0xFF
encrypted[11] = 0xFF
else:
prefix_start = 0
// Initialize padded_prefix for the starting prefix length
padded_prefix = pad_prefix(bytes16, prefix_start)
// Process each bit position sequentially
// prefix_len_bits is the number of bits processed from the MSB.
// Its range is [prefix_start, 128), excluding 128.
for prefix_len_bits from prefix_start to 128:
// Compute pseudorandom function with dual AES encryption
e1 = AES128_encrypt(K1, padded_prefix)
e2 = AES128_encrypt(K2, padded_prefix)
e = e1 ⊕ e2
// The PRF output is the least significant bit of e.
cipher_bit = get_bit(e, 0)
// Encrypt the current bit, processing from MSB to LSB.
// For IPv6, prefix_len_bits=0 encrypts bit 127;
// prefix_len_bits=1 encrypts bit 126, etc.
// For IPv4, prefix_len_bits=96 encrypts bit 31;
// prefix_len_bits=97 encrypts bit 30, etc.
bit_pos = 127 - prefix_len_bits
original_bit = get_bit(bytes16, bit_pos)
set_bit(encrypted, bit_pos, cipher_bit ^ original_bit)
// Prepare padded_prefix for next iteration
// Shift left by 1 bit and insert the next bit from bytes16
padded_prefix = shift_left_one_bit(padded_prefix)
set_bit(padded_prefix, 0, original_bit)
// Convert back to IP format
return bytes_16_to_ip(encrypted)
9.6.2. Decryption
function ipcrypt_pfx_decrypt(encrypted_ip, key):
// The key MUST be exactly 32 bytes (256 bits)
if length(key) != 32:
raise Error("Key must be 32 bytes")
// Convert encrypted IP to 16-byte representation
encrypted_bytes = convert_to_16_bytes(encrypted_ip)
// Split the key into two AES-128 keys
K1 = key[0:16]
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K2 = key[16:32]
// Initialize decrypted result with zeros
decrypted = [0] * 16
// For IPv4, start at bit 96, where the address starts.
if is_ipv4_mapped(encrypted_bytes):
prefix_start = 96
// Set up the IPv4-mapped IPv6 prefix
decrypted[10] = 0xFF
decrypted[11] = 0xFF
else:
prefix_start = 0
// Initialize padded_prefix for the starting prefix length
padded_prefix = pad_prefix(decrypted, prefix_start)
// Process each bit position sequentially
// prefix_len_bits is the number of bits processed from the MSB.
// Its range is [prefix_start, 128), excluding 128.
for prefix_len_bits from prefix_start to 128:
// Compute pseudorandom function with dual AES encryption
e1 = AES128_encrypt(K1, padded_prefix)
e2 = AES128_encrypt(K2, padded_prefix)
// e matches encryption because the prefix is the same.
e = e1 ⊕ e2
// The PRF output is the least significant bit of e.
cipher_bit = get_bit(e, 0)
// Decrypt the current bit, processing from MSB to LSB.
// For IPv6, prefix_len_bits=0 decrypts bit 127;
// prefix_len_bits=1 decrypts bit 126, etc.
// For IPv4, prefix_len_bits=96 decrypts bit 31;
// prefix_len_bits=97 decrypts bit 30, etc.
bit_pos = 127 - prefix_len_bits
encrypted_bit = get_bit(encrypted_bytes, bit_pos)
original_bit = cipher_bit ^ encrypted_bit
set_bit(decrypted, bit_pos, original_bit)
// Prepare padded_prefix for next iteration
// Shift left by 1 bit and insert the next bit from decrypted
padded_prefix = shift_left_one_bit(padded_prefix)
set_bit(padded_prefix, 0, original_bit)
// Convert back to IP format
return bytes_16_to_ip(decrypted)
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9.6.3. Helper Functions
The following helper functions are used in the ipcrypt-pfx
implementation:
function is_ipv4_mapped(bytes16):
// Return true for the IPv4-mapped IPv6 prefix
// (::ffff:0:0/96).
ipv4_mapped_prefix = [
0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
0x00, 0x00, 0x00, 0x00, 0xFF, 0xFF
]
return bytes16[0..12] == ipv4_mapped_prefix
function get_bit(data, position):
// Extract a bit from a 16-byte IPv6 address in network byte
// order.
// position: 0 = LSB of byte 15, 127 = MSB of byte 0
// Example: position 127 refers to bit 7 (MSB) of data[0]
// Example: position 0 refers to bit 0 (LSB) of data[15]
byte_index = 15 - (position / 8)
bit_index = position % 8
return (data[byte_index] >> bit_index) & 1
function set_bit(data, position, value):
// Set a bit in a 16-byte IPv6 address in network byte order.
// position: 0 = LSB of byte 15, 127 = MSB of byte 0
byte_index = 15 - (position / 8)
bit_index = position % 8
data[byte_index] |= ((value & 1) << bit_index)
function pad_prefix(data, prefix_len_bits):
// Specialized for the only two cases used: 0 and 96
// For prefix_len_bits=0, return a block with only bit 0 set
// (position 0 = LSB of byte 15).
// For prefix_len_bits=96, return the IPv4-mapped prefix with
// the separator at position 96.
if prefix_len_bits == 0:
// For IPv6 addresses starting from bit 0
padded_prefix = [0] * 16
// Set bit 0 (the LSB of byte 15).
padded_prefix[15] = 0x01
return padded_prefix
else if prefix_len_bits == 96:
// All IPv4 addresses share the same IPv4-mapped prefix
// (00...00 ffff), so always return the same value.
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padded_prefix = [0] * 16
// Set the separator at position 96 (bit 0 of byte 3).
padded_prefix[3] = 0x01
padded_prefix[14] = 0xFF // IPv4-mapped prefix
padded_prefix[15] = 0xFF // IPv4-mapped prefix
return padded_prefix
else:
raise Error(
"pad_prefix only supports prefix_len_bits of 0 or 96"
)
function shift_left_one_bit(data):
// Shift a 16-byte array one bit to the left
// Discard the most significant bit and shift in a zero from
// the right.
result = [0] * 16
carry = 0
// Process from the least significant byte (byte 15) to the
// most significant byte (byte 0).
for i from 15 down to 0:
// Shift the byte left by 1 with the previous carry.
result[i] = ((data[i] << 1) | carry) & 0xFF
// Extract the bit that will be carried to the next byte
carry = (data[i] >> 7) & 1
return result
9.7. Non-Deterministic Encryption using KIASU-BC (ipcrypt-nd)
9.7.1. Encryption
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function ipcrypt_nd_encrypt(ip_address, key):
if length(key) != 16:
raise Error("Key must be 16 bytes")
// Step 1: Generate random tweak (8 bytes)
tweak = random_bytes(8) // MUST be uniformly random
// Step 2: Convert IP to 16-byte representation
bytes16 = convert_to_16_bytes(ip_address)
// Step 3: Encrypt using key and tweak
ciphertext = KIASU_BC_encrypt(key, tweak, bytes16)
// Step 4: Concatenate tweak and ciphertext
// 8 bytes || 16 bytes = 24 bytes total
result = concatenate(tweak, ciphertext)
return result
9.7.2. Decryption
function ipcrypt_nd_decrypt(ciphertext, key):
// Step 1: Split ciphertext into tweak and encrypted IP
tweak = ciphertext[0:8] // First 8 bytes
encrypted_ip = ciphertext[8:24] // Remaining 16 bytes
// Step 2: Decrypt using key and tweak
bytes16 = KIASU_BC_decrypt(key, tweak, encrypted_ip)
// Step 3: Convert back to IP address
ip_address = bytes_16_to_ip(bytes16)
return ip_address
9.8. Non-Deterministic Encryption using AES-XTS (ipcrypt-ndx)
9.8.1. Encryption
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function ipcrypt_ndx_encrypt(ip_address, key):
if length(key) != 32:
raise Error("Key must be 32 bytes (two AES-128 keys)")
// Step 1: Generate random tweak (16 bytes)
tweak = random_bytes(16) // MUST be uniformly random
// Step 2: Convert IP to 16-byte representation
bytes16 = convert_to_16_bytes(ip_address)
// Step 3: Encrypt using key and tweak
ciphertext = AES_XTS_encrypt(key, tweak, bytes16)
// Step 4: Concatenate tweak and ciphertext
// 16 bytes || 16 bytes = 32 bytes total
result = concatenate(tweak, ciphertext)
return result
9.8.2. Decryption
function ipcrypt_ndx_decrypt(ciphertext, key):
// Step 1: Split ciphertext into tweak and encrypted IP
tweak = ciphertext[0:16] // First 16 bytes
encrypted_ip = ciphertext[16:32] // Remaining 16 bytes
// Step 2: Decrypt using key and tweak
bytes16 = AES_XTS_decrypt(key, tweak, encrypted_ip)
// Step 3: Convert back to IP address
ip_address = bytes_16_to_ip(bytes16)
return ip_address
9.8.3. Helper Functions for AES-XTS
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function AES_XTS_encrypt(key, tweak, block):
// Split the key into two halves
K1, K2 = split_key(key)
// Encrypt the tweak with the second half of the key
ET = AES128_encrypt(K2, tweak)
// Encrypt the block: AES128_encrypt(K1, block ⊕ ET) ⊕ ET
return AES128_encrypt(K1, block ⊕ ET) ⊕ ET
function AES_XTS_decrypt(key, tweak, block):
// Split the key into two halves
K1, K2 = split_key(key)
// Encrypt the tweak with the second half of the key
ET = AES128_encrypt(K2, tweak)
// Decrypt the block: AES128_decrypt(K1, block ⊕ ET) ⊕ ET
return AES128_decrypt(K1, block ⊕ ET) ⊕ ET
9.9. KIASU-BC Implementation Guide
This section provides a guide for implementing the KIASU-BC tweakable
block cipher used in ipcrypt-nd.
9.9.1. Overview
KIASU-BC extends AES-128 by incorporating an 8-byte tweak into each
round. The tweak is padded to 16 bytes and XORed with the round key
at each round.
9.9.2. Tweak Padding
The 8-byte tweak is padded to 16 bytes using the following method:
1. Split the 8-byte tweak into four 2-byte pairs
2. Place each 2-byte pair at the start of each 4-byte group
3. Fill the remaining 2 bytes of each group with zeros
Example:
8-byte tweak: [T0 T1 T2 T3 T4 T5 T6 T7]
16-byte padded: [T0 T1 00 00 T2 T3 00 00 T4 T5 00 00 T6 T7 00 00]
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9.9.3. Round Structure
Each round of KIASU-BC consists of the following standard AES
operations:
1. SubBytes: Apply the AES S-box to each byte of the state
2. ShiftRows: Rotate each row of the state matrix
3. MixColumns: Mix the columns of the state matrix (except in the
final round)
4. AddRoundKey: XOR the state with the round key and padded tweak
Details about these operations are provided in [FIPS-197].
9.9.4. Key Schedule
The key schedule follows the standard AES-128 key expansion:
1. The initial key is expanded into 11 round keys
2. Each round key is XORed with the padded tweak before use
3. The first round key is used in the initial AddRoundKey operation
9.9.5. Implementation Steps
1. Key Expansion:
* Expand the 16-byte key into 11 round keys using the standard
AES key schedule
* Each round key is 16 bytes
2. Tweak Processing:
* Pad the 8-byte tweak to 16 bytes as described above
* XOR the padded tweak with each round key before use
3. Encryption Process:
* Perform initial AddRoundKey with the first tweaked round key
* For rounds 1-9:
- SubBytes
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- ShiftRows
- MixColumns
- AddRoundKey (with tweaked round key)
* For round 10 (final round):
- SubBytes
- ShiftRows
- AddRoundKey (with tweaked round key)
9.9.6. Example Implementation
The following pseudocode illustrates the core operations of KIASU-BC:
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function pad_tweak(tweak):
// Input: 8-byte tweak
// Output: 16-byte padded tweak
padded = [0] * 16
for i in range(0, 4):
padded[i*4] = tweak[i*2]
padded[i*4+1] = tweak[i*2+1]
return padded
function kiasu_bc_encrypt(key, tweak, plaintext):
// Input: 16-byte key, 8-byte tweak, 16-byte plaintext
// Output: 16-byte ciphertext
// Expand key and pad tweak
round_keys = expand_key(key)
padded_tweak = pad_tweak(tweak)
// Initial round
state = plaintext
state = add_round_key(state, round_keys[0] ^ padded_tweak)
// Main rounds
for round in range(1, 10):
state = sub_bytes(state)
state = shift_rows(state)
state = mix_columns(state)
state = add_round_key(
state, round_keys[round] ^ padded_tweak)
// Final round
state = sub_bytes(state)
state = shift_rows(state)
state = add_round_key(state, round_keys[10] ^ padded_tweak)
return state
Key and tweak sizes for each variant:
* ipcrypt-deterministic: Key: 16 bytes (128 bits), no tweak, Output:
16 bytes
* ipcrypt-pfx: Key: 32 bytes (256 bits, split into two independent
AES-128 keys), no external tweak (uses prefix as cryptographic
context), Output: 4 bytes for IPv4, 16 bytes for IPv6
* ipcrypt-nd: Key: 16 bytes (128 bits), Tweak: 8 bytes (64 bits),
Output: 24 bytes
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* ipcrypt-ndx: Key: 32 bytes (256 bits, split into two AES-128
keys), Tweak: 16 bytes (128 bits), Output: 32 bytes
10. References
10.1. Normative References
[FIPS-197] NIST, "Advanced Encryption Standard (AES)", NIST FIPS 197,
DOI 10.6028/NIST.FIPS.197-upd1, May 2023,
<https://doi.org/10.6028/NIST.FIPS.197-upd1>.
[NIST-SP-800-38G]
NIST, "Recommendation for Block Cipher Modes of Operation:
Methods for Format-Preserving Encryption", NIST SP
800-38G, March 2016,
<https://nvlpubs.nist.gov/nistpubs/SpecialPublications/
NIST.SP.800-38G.pdf>.
[RFC4086] Eastlake 3rd, D., Schiller, J., and S. Crocker,
"Randomness Requirements for Security", BCP 106, RFC 4086,
DOI 10.17487/RFC4086, June 2005,
<https://www.rfc-editor.org/rfc/rfc4086>.
[RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 4291, DOI 10.17487/RFC4291, February
2006, <https://www.rfc-editor.org/rfc/rfc4291>.
[RFC5869] Krawczyk, H. and P. Eronen, "HMAC-based Extract-and-Expand
Key Derivation Function (HKDF)", RFC 5869,
DOI 10.17487/RFC5869, May 2010,
<https://www.rfc-editor.org/rfc/rfc5869>.
[RFC5952] Kawamura, S. and M. Kawashima, "A Recommendation for IPv6
Address Text Representation", RFC 5952,
DOI 10.17487/RFC5952, August 2010,
<https://www.rfc-editor.org/rfc/rfc5952>.
[RFC6973] Cooper, A., Tschofenig, H., Aboba, B., Peterson, J.,
Morris, J., Hansen, M., and R. Smith, "Privacy
Considerations for Internet Protocols", RFC 6973,
DOI 10.17487/RFC6973, July 2013,
<https://www.rfc-editor.org/rfc/rfc6973>.
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[RFC7624] Barnes, R., Schneier, B., Jennings, C., Hardie, T.,
Trammell, B., Huitema, C., and D. Borkmann,
"Confidentiality in the Face of Pervasive Surveillance: A
Threat Model and Problem Statement", RFC 7624,
DOI 10.17487/RFC7624, August 2015,
<https://www.rfc-editor.org/rfc/rfc7624>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/rfc/rfc8174>.
10.2. Informative References
[BRW2005] Black, J. and P. Rogaway, "Format-Preserving Encryption",
CT-RSA 2002, LNCS 2271, pp. 114-130, 8 February 2002,
<https://www.cs.ucdavis.edu/~rogaway/papers/subset.pdf>.
[DEOXYS-TBC]
Jean, J., Nikolić, I., Peyrin, T., and Y. Seurin, "The
Deoxys AEAD Family", Journal of Cryptology 34, 31 (2021),
10 June 2021,
<https://thomaspeyrin.github.io/web/assets/docs/papers/
Jean-etal-JoC2021.pdf>.
[FAST] Betul Durak, F., Horst, H., and S. Vaudenay, "FAST:
Format-Preserving Encryption via Shortened AES Tweakable
Block Cipher", Cryptology ePrint Archive Paper 2021/1171,
14 September 2021,
<https://eprint.iacr.org/2021/1171.pdf>.
[IEEE-P1619]
IEEE, "IEEE Standard for Cryptographic Protection of Data
on Block-Oriented Storage Devices", IEEE 1619-2007, 4
March 2008,
<https://ieeexplore.ieee.org/document/4493450>.
[IPCRYPT] Denis, F., "IPCrypt: Optimal, Practical Encryption of IP
Addresses for Privacy and Measurement", Cryptology ePrint
Archive Paper 2025/1689, 9 January 2025,
<https://eprint.iacr.org/2025/1689>.
[KIASU-BC] Jean, J., Nikolić, I., and T. Peyrin, "Tweaks and Keys for
Block Ciphers: the TWEAKEY Framework", ASIACRYPT 2014,
LNCS 8874, pp. 274-288, December 2014,
<https://eprint.iacr.org/2014/831.pdf>.
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[LRW2002] Liskov, M., Rivest, R., and D. Wagner, "Tweakable Block
Ciphers", CRYPTO 2002, LNCS 2442, pp. 31-46, 18 August
2002,
<https://people.csail.mit.edu/rivest/pubs/LRW02.pdf>.
[PATARIN-2008]
Patarin, J., "A Proof of Security in O(2^n) for the Xor of
Two Random Permutations", ICITS 2008, LNCS 5155, pp.
232-248, 2008, <https://link.springer.com/
chapter/10.1007/978-3-540-85093-9_22>.
[PATARIN-H-COEFFICIENTS]
Patarin, J., "The 'Coefficients H' Technique", SAC 2008,
LNCS 5381, pp. 328-345, 2009, <https://link.springer.com/
chapter/10.1007/978-3-642-04159-4_21>.
[RSSAC040] ICANN RSSAC, "RSSAC040: Recommendations on Anonymization
Processes for Source IP Addresses Submitted for Future
Analysis", ICANN RSSAC RSSAC040, 7 August 2018,
<https://www.icann.org/en/system/files/files/rssac-
040-07aug18-en.pdf>.
[SKINNY] Beierle, C., Jean, J., Koelbl, S., Leander, G., Moradi,
A., Peyrin, T., Sasaki, Y., Sasdrich, P., and S. Meng Sim,
"The SKINNY Family of Block Ciphers and its Low-Latency
Variant MANTIS", CRYPTO 2016, LNCS 9815, pp. 123-153, 14
August 2016, <https://eprint.iacr.org/2016/660.pdf>.
[SUM-OF-PRPS]
Lucks, S., "The Sum of PRPs Is a Secure PRF", EUROCRYPT
2000, LNCS 1807, pp. 470-484, 14 May 2000,
<https://link.springer.com/content/
pdf/10.1007/3-540-45539-6_34.pdf>.
Appendix A. Test Vectors
This appendix provides test vectors for the ipcrypt variants. Each
test vector includes the key, input IP address, and encrypted output.
Non-deterministic variants include the tweak value.
Implementations MUST verify their correctness against these test
vectors before deployment.
A.1. ipcrypt-deterministic Test Vectors
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# Test vector 1
Key: 0123456789abcdeffedcba9876543210
Input IP: 0.0.0.0
Encrypted IP: bde9:6789:d353:824c:d7c6:f58a:6bd2:26eb
# Test vector 2
Key: 1032547698badcfeefcdab8967452301
Input IP: 255.255.255.255
Encrypted IP: aed2:92f6:ea23:58c3:48fd:8b8:74e8:45d8
# Test vector 3
Key: 2b7e151628aed2a6abf7158809cf4f3c
Input IP: 192.0.2.1
Encrypted IP: 1dbd:c1b9:fff1:7586:7d0b:67b4:e76e:4777
A.2. ipcrypt-pfx Test Vectors
The following test vectors demonstrate the prefix-preserving property
of ipcrypt-pfx. Addresses from the same network produce encrypted
addresses that share a common encrypted prefix.
A.2.1. Basic Test Vectors
# Test vector 1 (IPv4)
Key: 0123456789abcdeffedcba9876543210
1032547698badcfeefcdab8967452301
Input IP: 0.0.0.0
Encrypted IP: 151.82.155.134
# Test vector 2 (IPv4)
Key: 0123456789abcdeffedcba9876543210
1032547698badcfeefcdab8967452301
Input IP: 255.255.255.255
Encrypted IP: 94.185.169.89
# Test vector 3 (IPv4)
Key: 0123456789abcdeffedcba9876543210
1032547698badcfeefcdab8967452301
Input IP: 192.0.2.1
Encrypted IP: 100.115.72.131
# Test vector 4 (IPv6)
Key: 0123456789abcdeffedcba9876543210
1032547698badcfeefcdab8967452301
Input IP: 2001:db8::1
Encrypted IP: c180:5dd4:2587:3524:30ab:fa65:6ab6:f88
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A.2.2. Prefix-Preserving Test Vectors
These test vectors demonstrate the prefix-preserving property.
Addresses from the same network share common encrypted prefixes at
the corresponding prefix length.
# IPv4 addresses from same /24 network (10.0.0.0/24)
Key: 2b7e151628aed2a6abf7158809cf4f3c
a9f5ba40db214c3798f2e1c23456789a
Input IP: 10.0.0.47
Encrypted IP: 19.214.210.244
Key: 2b7e151628aed2a6abf7158809cf4f3c
a9f5ba40db214c3798f2e1c23456789a
Input IP: 10.0.0.129
Encrypted IP: 19.214.210.80
Key: 2b7e151628aed2a6abf7158809cf4f3c
a9f5ba40db214c3798f2e1c23456789a
Input IP: 10.0.0.234
Encrypted IP: 19.214.210.30
# IPv4 addresses from same /16 but different /24 networks
Key: 2b7e151628aed2a6abf7158809cf4f3c
a9f5ba40db214c3798f2e1c23456789a
Input IP: 172.16.5.193
Encrypted IP: 210.78.229.136
Key: 2b7e151628aed2a6abf7158809cf4f3c
a9f5ba40db214c3798f2e1c23456789a
Input IP: 172.16.97.42
Encrypted IP: 210.78.179.241
Key: 2b7e151628aed2a6abf7158809cf4f3c
a9f5ba40db214c3798f2e1c23456789a
Input IP: 172.16.248.177
Encrypted IP: 210.78.121.215
# IPv6 addresses from same /64 network (2001:db8::/64)
Key: 2b7e151628aed2a6abf7158809cf4f3c
a9f5ba40db214c3798f2e1c23456789a
Input IP: 2001:db8::a5c9:4e2f:bb91:5a7d
Encrypted IP: 7cec:702c:1243:f70:1956:125:b9bd:1aba
Key: 2b7e151628aed2a6abf7158809cf4f3c
a9f5ba40db214c3798f2e1c23456789a
Input IP: 2001:db8::7234:d8f1:3c6e:9a52
Encrypted IP: 7cec:702c:1243:f70:a3ef:c8e:95c1:cd0d
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Key: 2b7e151628aed2a6abf7158809cf4f3c
a9f5ba40db214c3798f2e1c23456789a
Input IP: 2001:db8::f1e0:937b:26d4:8c1a
Encrypted IP: 7cec:702c:1243:f70:443c:c8e:6a62:b64d
# IPv6 addresses from same /32 but different /48 networks
Key: 2b7e151628aed2a6abf7158809cf4f3c
a9f5ba40db214c3798f2e1c23456789a
Input IP: 2001:db8:3a5c:0:e7d1:4b9f:2c8a:f673
Encrypted IP: 7cec:702c:3503:bef:e616:96bd:be33:a9b9
Key: 2b7e151628aed2a6abf7158809cf4f3c
a9f5ba40db214c3798f2e1c23456789a
Input IP: 2001:db8:9f27:0:b4e2:7a3d:5f91:c8e6
Encrypted IP: 7cec:702c:a504:b74e:194a:3d90:b047:2d1a
Key: 2b7e151628aed2a6abf7158809cf4f3c
a9f5ba40db214c3798f2e1c23456789a
Input IP: 2001:db8:d8b4:0:193c:a5e7:8b2f:46d1
Encrypted IP: 7cec:702c:f840:aa67:1b8:e84f:ac9d:77fb
A.3. ipcrypt-nd Test Vectors
# Test vector 1
Key: 0123456789abcdeffedcba9876543210
Input IP: 0.0.0.0
Tweak: 08e0c289bff23b7c
Output: 08e0c289bff23b7cb349aadfe3bcef56221c384c7c217b16
# Test vector 2
Key: 1032547698badcfeefcdab8967452301
Input IP: 192.0.2.1
Tweak: 21bd1834bc088cd2
Output: 21bd1834bc088cd2e5e1fe55f95876e639faae2594a0caad
# Test vector 3
Key: 2b7e151628aed2a6abf7158809cf4f3c
Input IP: 2001:db8::1
Tweak: b4ecbe30b70898d7
Output: b4ecbe30b70898d7553ac8974d1b4250eafc4b0aa1f80c96
A.4. ipcrypt-ndx Test Vectors
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# Test vector 1
Key: 0123456789abcdeffedcba9876543210
1032547698badcfeefcdab8967452301
Input IP: 0.0.0.0
Tweak: 21bd1834bc088cd2b4ecbe30b70898d7
Output: 21bd1834bc088cd2b4ecbe30b70898d7
82db0d4125fdace61db35b8339f20ee5
# Test vector 2
Key: 1032547698badcfeefcdab8967452301
0123456789abcdeffedcba9876543210
Input IP: 192.0.2.1
Tweak: 08e0c289bff23b7cb4ecbe30b70898d7
Output: 08e0c289bff23b7cb4ecbe30b70898d7
766a533392a69edf1ad0d3ce362ba98a
# Test vector 3
Key: 2b7e151628aed2a6abf7158809cf4f3c
3c4fcf098815f7aba6d2ae2816157e2b
Input IP: 2001:db8::1
Tweak: 21bd1834bc088cd2b4ecbe30b70898d7
Output: 21bd1834bc088cd2b4ecbe30b70898d7
6089c7e05ae30c2d10ca149870a263e4
For non-deterministic variants, the tweak values shown are examples.
Tweaks MUST be uniformly random for each encryption operation.
IANA Considerations
This document does not require any IANA actions.
Acknowledgments
We would like to thank the members of the Internet Engineering Task
Force (IETF) and the cryptographic community for their helpful
comments on this work. Tobias Fiebig conducted a detailed review of
this document. Additional valuable feedback was provided by Eliot
Lear. Assistance with implementation aspects was kindly provided by
Wu Tingfeng.
Authors' Addresses
Christopher Barber
Trim
Email: cbarbernash@gmail.com
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Frank Denis
Fastly Inc.
Email: fde@00f.net
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