Methods for IP Address Encryption and Obfuscation
draft-denis-ipcrypt-05
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draft-denis-ipcrypt-05
Network Working Group F. Denis
Internet-Draft Fastly Inc.
Intended status: Informational 6 August 2025
Expires: 7 February 2026
Methods for IP Address Encryption and Obfuscation
draft-denis-ipcrypt-05
Abstract
This document specifies methods for encrypting and obfuscating IP
addresses for privacy-preserving storage, logging, and analytics.
These encrypted addresses enable data analysis while protecting user
privacy, addressing concerns raised in [RFC6973] and [RFC7258]
regarding pervasive monitoring.
Three concrete instantiations are defined: ipcrypt-deterministic
provides deterministic, format-preserving encryption, while ipcrypt-
nd and ipcrypt-ndx introduce randomness to prevent correlation. All
methods are reversible with the encryption key.
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
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on 7 February 2026.
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Copyright Notice
Copyright (c) 2025 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 . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Use Cases and Motivations . . . . . . . . . . . . . . . . 3
1.2. Relationship to IETF Work . . . . . . . . . . . . . . . . 4
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. IP Address Conversion . . . . . . . . . . . . . . . . . . . . 5
3.1. Converting to a 16-Byte Representation . . . . . . . . . 5
3.1.1. IPv6 Addresses . . . . . . . . . . . . . . . . . . . 6
3.1.2. IPv4 Addresses . . . . . . . . . . . . . . . . . . . 6
3.2. Converting from a 16-Byte Representation to an IP
Address . . . . . . . . . . . . . . . . . . . . . . . . . 6
4. Generic Constructions . . . . . . . . . . . . . . . . . . . . 6
5. Deterministic Encryption . . . . . . . . . . . . . . . . . . 7
5.1. ipcrypt-deterministic . . . . . . . . . . . . . . . . . . 7
5.2. Format Preservation . . . . . . . . . . . . . . . . . . . 8
6. Non-Deterministic Encryption . . . . . . . . . . . . . . . . 8
6.1. Encryption Process . . . . . . . . . . . . . . . . . . . 8
6.2. Decryption Process . . . . . . . . . . . . . . . . . . . 9
6.3. Output Format and Encoding . . . . . . . . . . . . . . . 9
6.4. Concrete Instantiations . . . . . . . . . . . . . . . . . 9
6.4.1. ipcrypt-nd (KIASU-BC) . . . . . . . . . . . . . . . . 10
6.4.2. ipcrypt-ndx (AES-XTS) . . . . . . . . . . . . . . . . 10
6.4.3. Comparison of Modes . . . . . . . . . . . . . . . . . 11
6.5. Alternatives to Random Tweaks . . . . . . . . . . . . . . 11
7. Security Considerations . . . . . . . . . . . . . . . . . . . 11
7.1. Deterministic Mode Security . . . . . . . . . . . . . . . 12
7.2. Non-Deterministic Mode Security . . . . . . . . . . . . . 12
7.3. Implementation Security . . . . . . . . . . . . . . . . . 12
7.4. Key Management Considerations . . . . . . . . . . . . . . 12
8. Implementation Details . . . . . . . . . . . . . . . . . . . 13
8.1. Visual Diagrams . . . . . . . . . . . . . . . . . . . . . 13
8.1.1. IPv4 Address Conversion Diagram . . . . . . . . . . . 13
8.1.2. Deterministic Encryption Flow . . . . . . . . . . . . 13
8.1.3. Non-Deterministic Encryption Flow (ipcrypt-nd) . . . 14
8.1.4. Non-Deterministic Encryption Flow (ipcrypt-ndx) . . . 14
8.2. IPv4 Address Conversion . . . . . . . . . . . . . . . . . 15
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8.3. IPv6 Address Conversion . . . . . . . . . . . . . . . . . 15
8.4. Conversion from a 16-Byte Array to an IP Address . . . . 16
8.5. Deterministic Encryption (ipcrypt-deterministic) . . . . 16
8.6. Non-Deterministic Encryption using KIASU-BC
(ipcrypt-nd) . . . . . . . . . . . . . . . . . . . . . . 17
8.7. Non-Deterministic Encryption using AES-XTS
(ipcrypt-ndx) . . . . . . . . . . . . . . . . . . . . . . 18
8.8. KIASU-BC Implementation Guide . . . . . . . . . . . . . . 19
8.8.1. Overview . . . . . . . . . . . . . . . . . . . . . . 19
8.8.2. Tweak Padding . . . . . . . . . . . . . . . . . . . . 19
8.8.3. Round Structure . . . . . . . . . . . . . . . . . . . 20
8.8.4. Key Schedule . . . . . . . . . . . . . . . . . . . . 20
8.8.5. Implementation Steps . . . . . . . . . . . . . . . . 20
8.8.6. Example Implementation . . . . . . . . . . . . . . . 21
9. Implementation Status . . . . . . . . . . . . . . . . . . . . 22
10. Licensing . . . . . . . . . . . . . . . . . . . . . . . . . . 23
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 24
11.1. Normative References . . . . . . . . . . . . . . . . . . 24
11.2. Informative References . . . . . . . . . . . . . . . . . 25
Appendix A. Test Vectors . . . . . . . . . . . . . . . . . . . . 26
A.1. ipcrypt-deterministic Test Vectors . . . . . . . . . . . 26
A.2. ipcrypt-nd Test Vectors . . . . . . . . . . . . . . . . . 26
A.3. ipcrypt-ndx Test Vectors . . . . . . . . . . . . . . . . 27
IANA Considerations . . . . . . . . . . . . . . . . . . . . . . . 27
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 27
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 27
1. Introduction
This document specifies methods for the encryption and obfuscation of
IP addresses for both operational use and privacy preservation. The
objective is to enable network operators, researchers, and privacy
advocates to share or analyze data while protecting sensitive address
information.
This work addresses concerns raised in [RFC7624] regarding
confidentiality in the face of pervasive surveillance. The security
properties of these methods are discussed throughout this document
and summarized in Section 7.
1.1. Use Cases and Motivations
IP addresses are personally identifiable information (PII). While
generic encryption systems can protect them, the specialized methods
described here offer significant advantages with well-defined
security guarantees:
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* *Efficiency and Compactness:* All variants operate on exactly 128
bits, providing single-block encryption speed. Non-deterministic
variants add only 8-16 bytes of tweak overhead compared to
arbitrary expansion in generic encryption systems. This enables
processing billions of addresses at network speeds.
* *High Usage Limits:* Non-deterministic variants support extensive
operations per key - approximately 4 billion for ipcrypt-nd and 18
quintillion for ipcrypt-ndx - far exceeding typical cryptographic
limits while maintaining compact outputs.
* *Format Preservation (Deterministic):* The ipcrypt-deterministic
variant produces valid IP addresses, enabling seamless integration
with existing network tools that validate IP formats (see
Section 5.2).
* *Interoperability:* By following the recommendations from this
specification, implementations can reliably encrypt and decrypt IP
addresses in a compatible way across different systems and
vendors.
These specialized encryption methods unlock several critical use
cases:
* *Privacy Protection:* They prevent the exposure of sensitive user
information in logs, analytics data, and network measurements
([RFC6973]).
* *Correlation Attack Resistance:* While deterministic encryption
can reveal repeated inputs, the non-deterministic variants
leverage random tweaks to hide patterns and enhance
confidentiality (see Section 6).
* *Privacy-Preserving Analytics:* Encrypted IP addresses can be used
directly for operations such as counting unique clients, rate
limiting, or deduplication—without needing to reveal or access the
original values.
* *Seamless Third-Party Integration:* Encrypted IPs can act as
privacy-preserving identifiers when interacting with untrusted
services, cloud providers, or external platforms.
For implementation guidelines and practical examples, see Section 8.
1.2. Relationship to IETF Work
_This section is to be removed before publishing as an RFC._
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This document does not conflict with any active IETF working group
efforts. While the IETF has produced several RFCs related to privacy
([RFC6973], [RFC7258], [RFC7624]), there is no current
standardization effort for IP address encryption methods. This
specification complements existing IETF privacy guidance by providing
concrete implementation methods.
The cryptographic primitives used (AES, format-preserving encryption)
align with IETF cryptographic recommendations, and the document
follows IETF formatting and terminology conventions where applicable.
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].
* *16-Byte Representation:* A fixed-length representation used for
both IPv4 (via IPv4-mapped IPv6) and IPv6 addresses.
* *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.
* *(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. This conversion is necessary to operate a
128-bit cipher on both IPv4 and IPv6 addresses.
3.1. Converting to a 16-Byte Representation
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3.1.1. IPv6 Addresses
IPv6 addresses are natively 128 bits and are converted directly using
network byte order (big-endian) 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]
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]
3.2. Converting from a 16-Byte Representation to an IP Address
The conversion algorithm is as follows:
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 6)
* 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-BC* (see [DEOXYS-BC])
* *KIASU-BC* (see Section 8.8 for implementation details)
* *AES-XTS* (see Section 6.4.2 for usage)
Implementers MUST choose a cipher that meets the required security
properties and 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. All instantiations
documented in this specification (ipcrypt-deterministic, ipcrypt-nd,
and ipcrypt-ndx) are invertible - encrypted IP addresses can be
decrypted back to their original values using the same key. For non-
deterministic modes, the tweak must be preserved along with the
ciphertext to enable decryption.
For implementation details, see Section 8.
5.1. ipcrypt-deterministic
The ipcrypt-deterministic instantiation employs AES-128 in a single-
block operation. The key MUST be exactly 16 bytes (128 bits) in
length. Since AES-128 is a permutation, every distinct 16-byte input
maps to a unique 16-byte ciphertext, preserving the IP address
format.
For test vectors, see Appendix A.1.
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+---------------------+
| IP Address |
| (IPv4 or IPv6) |
+---------------------+
|
v
+---------------------+
| Convert to 16 Bytes |
+---------------------+
|
v
+---------------------+
| AES128 Encrypt |
| (Single Block) |
+---------------------+
|
v
+---------------------+
| 16-Byte Output |
+---------------------+
|
v
+---------------------+
| Convert to IP Format|
+---------------------+
5.2. Format Preservation
* If the 16-byte ciphertext begins with an IPv4-mapped prefix, it
MUST be rendered as a dotted-decimal IPv4 address.
* Otherwise, it is interpreted as an IPv6 address.
To ensure IPv4 format preservation, implementers MUST consider using
cycle-walking (repeatedly encrypting until a valid IPv4-mapped
address is obtained), a 32-bit random permutation, or a Format-
Preserving Encryption (FPE) mode as specified in [NIST-SP-800-38G].
6. Non-Deterministic Encryption
Non-deterministic encryption leverages a tweakable block cipher
together with a random tweak. For implementation details, see
Section 8.
6.1. Encryption Process
The encryption process for non-deterministic modes consists of the
following steps:
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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.
This allows the same tweak to be used for decryption.
6.2. Decryption Process
The decryption process consists of the following steps:
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. Such collisions
are benign when they occur with different inputs. 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
discussed below apply per cryptographic key; rotating keys can extend
secure usage beyond these bounds.
6.3. Output Format and Encoding
The output of non-deterministic encryption is binary data. For
applications that require text representation (e.g., logging, JSON
encoding, or text-based protocols), the binary output MUST be
encoded. Common encoding options include hexadecimal and Base64.
The choice of encoding is application-specific and outside the scope
of this specification. However, implementations SHOULD document
their chosen encoding method clearly.
6.4. 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 [XTS-AES] 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.
For test vectors, see Appendix A.2 and Appendix A.3.
6.4.1. ipcrypt-nd (KIASU-BC)
The ipcrypt-nd instantiation uses the KIASU-BC tweakable block cipher
with an 8-byte (64-bit) tweak. For implementation details, see
Section 8.8. 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 for a
specific tweak value after about 2^(64/2) = 2^32 operations
(approximately 4 billion operations). If an (input, tweak) collision
occurs, it indicates that the same input was processed with that
tweak without revealing the input’s value.
These collision bounds apply per cryptographic key. By rotating keys
regularly, secure usage can be extended well beyond these bounds.
The effective security is determined by the underlying block cipher’s
strength.
For test vectors, see Appendix A.2.
6.4.2. 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.
For AES-XTS encryption of a single block, the computation avoids the
sequential tweak calculations required in full XTS mode. Independent
sampling of a 16-byte tweak results in an expected collision after
about 2^(128/2) = 2^64 operations (approximately 18 quintillion
operations).
As with ipcrypt-nd, an (input, tweak) collision reveals repetition
without compromising the input value. These limits are per key, and
regular key rotation further extends secure usage. The effective
security is governed by the strength of AES-128 (approximately 2^128
operations).
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6.4.3. Comparison of Modes
* *Deterministic (ipcrypt-deterministic):* Produces a 16-byte
output; preserves format but reveals repeated inputs.
* *Non-Deterministic:*
- *ipcrypt-nd (KIASU-BC):* Produces a 24-byte output using an
8-byte tweak; (input, tweak) collisions reveal repeated inputs
(with the same tweak) but not their values. Expected collision
after approximately 4 billion operations per key.
- *ipcrypt-ndx (AES-XTS):* Produces a 32-byte output using a
16-byte tweak; supports higher secure operation counts per key.
Expected collision after approximately 18 quintillion
operations per key.
6.5. Alternatives to Random Tweaks
While this specification recommends the use of uniformly random
tweaks for non-deterministic encryption, implementers may consider
alternative approaches:
* *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.
* *UUIDs:* 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 is a concern with random tweaks, the use
of random tweaks remains the recommended and most practical approach,
offering the best tradeoffs for most real-world use cases.
7. Security Considerations
The ipcrypt constructions focus solely on confidentiality and do not
provide integrity. This means that IP addresses in an ordered
sequence can be partially removed, duplicated, reordered, or blindly
altered by an active adversary. Applications that require sequences
of encrypted IP addresses that cannot be modified must apply an
authentication scheme over the entire sequence, such as an HMAC
construction, a keyed hash function, or a public key signature. This
is outside the scope of this specification, but implementers should
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be aware that additional authentication mechanisms are required if
protection against active adversaries is needed.
7.1. Deterministic Mode Security
A permutation ensures distinct inputs yield distinct outputs.
However, repeated inputs result in identical ciphertexts, thereby
revealing repetition.
This property makes deterministic encryption suitable for
applications where format preservation is required, but linkability
of repeated inputs is acceptable.
7.2. Non-Deterministic Mode Security
The inclusion of a random tweak ensures that encrypting the same
input generally produces different outputs. In cases where an
(input, tweak) collision occurs, an attacker learns only that the
same input was processed with that tweak, not the value of the input
itself.
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.
7.3. 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
7.4. Key Management Considerations
This specification focuses on the cryptographic transformations and
does not mandate specific key management practices. However,
implementers MUST ensure:
1. Keys are generated using cryptographically secure random number
generators (see [RFC4086])
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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
For high-volume deployments processing billions of IP addresses,
regular key rotation (e.g., monthly or quarterly) is RECOMMENDED to
stay well within the security bounds discussed in this document.
8. Implementation Details
This section provides detailed pseudocode and implementation guidance
for the key operations described in this document.
8.1. Visual Diagrams
The following diagrams illustrate the key processes described in this
specification.
8.1.1. 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]
8.1.2. Deterministic Encryption Flow
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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
8.1.3. 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)
8.1.4. 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)
8.2. IPv4 Address Conversion
For a diagram of this conversion process, see Section 8.1.1.
function IPv4To16Bytes(ipv4_address):
// Split the IPv4 address into its octets
parts = ipv4_address.split(".")
if length(parts) != 4:
raise Error("Invalid IPv4 address")
// 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 part in parts:
bytes16.append(int(part))
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]
8.3. IPv6 Address Conversion
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function IPv6To16Bytes(ipv6_address):
// Parse the IPv6 address into eight 16-bit words.
words = parseIPv6(ipv6_address) // Expands shorthand notation and returns 8 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.
8.4. Conversion from a 16-Byte Array to an IP Address
function Bytes16ToIP(bytes16):
if length(bytes16) != 16:
raise Error("Invalid byte array")
// Check for the IPv4-mapped prefix
if bytes16[0:10] == [0x00]*10 and bytes16[10] == 0xFF and bytes16[11] == 0xFF:
ipv4_parts = []
for i from 12 to 15:
ipv4_parts.append(str(bytes16[i]))
ipv4_address = join(ipv4_parts, ".")
return ipv4_address
else:
words = []
for i from 0 to 15 step 2:
word = (bytes16[i] << 8) | bytes16[i+1]
words.append(format(word, "x"))
ipv6_address = join(words, ":")
return ipv6_address
8.5. Deterministic Encryption (ipcrypt-deterministic)
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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 = convertTo16Bytes(ip_address)
ciphertext = AES128_encrypt(key, bytes16)
encrypted_ip = Bytes16ToIP(ciphertext)
return encrypted_ip
function ipcrypt_deterministic_decrypt(encrypted_ip, key):
if length(key) != 16:
raise Error("Key must be 16 bytes")
bytes16 = convertTo16Bytes(encrypted_ip)
plaintext = AES128_decrypt(key, bytes16)
original_ip = Bytes16ToIP(plaintext)
return original_ip
8.6. Non-Deterministic Encryption using KIASU-BC (ipcrypt-nd)
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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 = convertTo16Bytes(ip_address)
// Step 3: Encrypt using key and tweak
ciphertext = KIASU_BC_encrypt(key, tweak, bytes16)
// Step 4: Concatenate tweak and ciphertext
result = concatenate(tweak, ciphertext) // 8 bytes || 16 bytes = 24 bytes total
return result
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 = Bytes16ToIP(bytes16)
return ip_address
8.7. Non-Deterministic Encryption using AES-XTS (ipcrypt-ndx)
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(block ⊕ ET, K1) ⊕ ET
return AES128_encrypt(K1, block ⊕ ET) ⊕ ET
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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 = convertTo16Bytes(ip_address)
// Step 3: Encrypt using key and tweak
ciphertext = AES_XTS_encrypt(key, tweak, bytes16)
// Step 4: Concatenate tweak and ciphertext
result = concatenate(tweak, ciphertext) // 16 bytes || 16 bytes = 32 bytes total
return result
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 = Bytes16ToIP(bytes16)
return ip_address
8.8. KIASU-BC Implementation Guide
This section provides a detailed guide for implementing the KIASU-BC
tweakable block cipher used in ipcrypt-nd. KIASU-BC is based on
AES-128 with modifications to incorporate a tweak.
8.8.1. 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 of the cipher. This construction is used in the
ipcrypt-nd instantiation.
8.8.2. 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
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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]
8.8.3. 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
For details about these operations, see [FIPS-197].
8.8.4. 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
8.8.5. 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
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* 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
- ShiftRows
- MixColumns
- AddRoundKey (with tweaked round key)
* For round 10 (final round):
- SubBytes
- ShiftRows
- AddRoundKey (with tweaked round key)
8.8.6. 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, 8, 2):
padded[i*2] = tweak[i]
padded[i*2+1] = tweak[i+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 - ipcrypt-nd: Key: 16 bytes (128 bits),
Tweak: 8 bytes (64 bits) - ipcrypt-ndx: Key: 32 bytes (256 bits, two
AES-128 keys), Tweak: 16 bytes (128 bits)
9. Implementation Status
_This section is to be removed before publishing as an RFC._
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This section records the status of known implementations of the
protocol defined by this specification at the time of posting of this
Internet-Draft, and is based on a proposal described in [RFC7942].
The description of implementations in this section is intended to
assist the Independent Submissions Editor in judging whether the
specification is suitable for publication.
Please note that the listing of any individual implementation here
does not imply endorsement. Furthermore, no effort has been spent to
verify the information presented here that was supplied by
contributors. This is not intended as, and must not be construed to
be, a catalog of available implementations or their features.
Multiple interoperable implementations of the schemes described in
this document have been developed:
* C implementation
* D implementation
* Go implementation
* Java implementation (maven package)
* JavaScript/TypeScript implementation (npm package)
* PHP implementation (Composer package)
* Python reference implementation
* Rust implementation (cargo package)
* Zig implementation
* Dart implementation (pub.dev package)
A comprehensive list of implementations and their test results can be
found at: https://ipcrypt-std.github.io/implementations/
All implementations pass the common test vectors specified in this
document, demonstrating interoperability across programming
languages.
10. Licensing
_This section is to be removed before publishing as an RFC._
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Implementations of the ipcrypt methods are freely available under
permissive open source licenses (MIT, BSD, or Apache 2.0) at the
repository listed in the Implementation Status section.
There are no known patent claims on these methods.
11. References
11.1. Normative References
[FIPS-197] NIST, "Advanced Encryption Standard (AES)", FIPS PUB 197,
26 November 2001, <https://nvlpubs.nist.gov/nistpubs/FIPS/
NIST.FIPS.197.pdf>.
[IEEE-P1619]
IEEE, "IEEE Standard for Cryptographic Protection of Data
on Block-Oriented Storage Devices", IEEE 1619-2007, 18
December 2007,
<https://standards.ieee.org/ieee/1619/2041/>.
[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>.
[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>.
[RFC7258] Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an
Attack", BCP 188, RFC 7258, DOI 10.17487/RFC7258, May
2014, <https://www.rfc-editor.org/rfc/rfc7258>.
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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>.
[RFC7942] Sheffer, Y. and A. Farrel, "Improving Awareness of Running
Code: The Implementation Status Section", BCP 205,
RFC 7942, DOI 10.17487/RFC7942, July 2016,
<https://www.rfc-editor.org/rfc/rfc7942>.
[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>.
11.2. Informative References
[BRW2005] Bellare, M., Rogaway, P., and D. Wagner, "Format-
Preserving Encryption", CRYPTO 2005, 2005,
<https://www.cs.ucdavis.edu/~rogaway/papers/subset.pdf>.
[DEOXYS-BC]
Jean, J., Nikolić, I., and T. Peyrin, "Deoxys-BC: A Highly
Secure Tweakable Block Cipher", Cryptology ePrint
Archive Paper 2014/427, 2014,
<https://eprint.iacr.org/2014/427>.
[IPCRYPT2] Denis, F., "ipcrypt2: IP address encryption/obfuscation
tool", 2025, <https://github.com/jedisct1/ipcrypt2>.
[KIASU-BC] Jean, J., Nikolić, I., and T. Peyrin, "Tweaks and Keys for
Block Ciphers: the TWEAKEY Framework", Cryptology ePrint
Archive Paper 2014/831, 2014,
<https://eprint.iacr.org/2014/831>.
[LRW2002] Liskov, M., Rivest, R., and D. Wagner, "Tweakable Block
Ciphers", Fast Software Encryption 2002, 2002,
<https://www.cs.berkeley.edu/~daw/papers/tweak-
crypto02.pdf>.
[SKINNY] Beierle, C., Biryukov, A., Perrin, L., Udovenko, A.,
Velichkov, V., and Q. Wang, "The SKINNY Family of Block
Ciphers and its Low-Latency Variant MANTIS", CRYPTO 2016,
2016, <https://eprint.iacr.org/2016/660>.
[XTS-AES] Black, J., Dawson, E., Gueron, S., and P. Rogaway, "The
XTS-AES Mode for Disk Encryption", IEEE 1619-2007, 2010.
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Appendix A. Test Vectors
This appendix provides test vectors for all three variants of
ipcrypt. Each test vector includes the key, input IP address, and
encrypted output. For non-deterministic variants (ipcrypt-nd and
ipcrypt-ndx), the tweak value is also included.
Implementations MUST verify their correctness against these test
vectors before deployment.
A.1. ipcrypt-deterministic Test Vectors
# 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-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
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A.3. ipcrypt-ndx Test Vectors
# Test vector 1
Key: 0123456789abcdeffedcba98765432101032547698badcfeefcdab8967452301
Input IP: 0.0.0.0
Tweak: 21bd1834bc088cd2b4ecbe30b70898d7
Output: 21bd1834bc088cd2b4ecbe30b70898d782db0d4125fdace61db35b8339f20ee5
# Test vector 2
Key: 1032547698badcfeefcdab89674523010123456789abcdeffedcba9876543210
Input IP: 192.0.2.1
Tweak: 08e0c289bff23b7cb4ecbe30b70898d7
Output: 08e0c289bff23b7cb4ecbe30b70898d7766a533392a69edf1ad0d3ce362ba98a
# Test vector 3
Key: 2b7e151628aed2a6abf7158809cf4f3c3c4fcf098815f7aba6d2ae2816157e2b
Input IP: 2001:db8::1
Tweak: 21bd1834bc088cd2b4ecbe30b70898d7
Output: 21bd1834bc088cd2b4ecbe30b70898d76089c7e05ae30c2d10ca149870a263e4
For non-deterministic variants (ipcrypt-nd and ipcrypt-ndx), the
tweak values shown are examples. In practice, tweaks MUST be
uniformly random for each encryption operation.
IANA Considerations
This document does not require any IANA actions.
Acknowledgments
The author gratefully acknowledges the contributions and insightful
comments from members of the IETF and the broader cryptographic
community that have helped shape this specification.
Author's Address
Frank Denis
Fastly Inc.
Email: fde@00f.net
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