S/MIME Working Group J. Schaad Internet Draft Soaring Hawk Consulting Document: draft-ietf-smime-aes-alg-04.txt R. Housley Expires: July 2002 RSA Laboratories January 2002 Use of the AES Encryption Algorithm and RSA-OAEP Key Transport in CMS Status of this Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC 2026. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. 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." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. Comments or suggestions for improvement may be made on the "ietf- smime" mailing list, or directly to the author. Abstract This document specifies the conventions for using the Advanced Encryption Standard (AES) algorithm [AES] for encryption and the RSAES-OAEP key transport algorithm [PKCS#1v2.0] for key management with the Cryptographic Message Syntax (CMS) [CMS]. Conventions used in this document The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [MUSTSHOULD]. 1 Overview This document specifies the conventions for using the RSAES-OAEP key transport algorithm and Advanced Encryption Standard (AES) content encryption algorithm with the Cryptographic Message Syntax [CMS] enveloped-data and encrypted-data content types. Schaad, Housley 1 Use of the AES Algorithm in CMS February 2002 This document presents the use of the two algorithms together, since we anticipate that they will be used together. However, the two algorithms can be used independently. For example, RSA-OAEP could be used to transport Triple-DES keys, and AES keys could be distributed out-of-band for use with mail lists. CMS values are generated using ASN.1 [X.208-88], using the Basic Encoding Rules (BER) [X.209-88] and the Distinguished Encoding Rules (DER) [X.509-88]. 1.1 AES The Advanced Encryption Standard (AES) [AES] was developed to replace DES [DES]. The AES Federal Information Processing Standard (FIPS) Publication specifies a cryptographic algorithm for use by U.S. Government organizations. However, the AES will also be widely used by organizations, institutions, and individuals outside of the U.S. Government. Two researchers who developed and submitted the Rijndael algorithm for consideration are both cryptographers from Belgium: Dr. Joan Daemen of Proton World International and Dr. Vincent Rijmen, a postdoctoral researcher in the Electrical Engineering Department of Katholieke Universiteit Leuven. The National Institute of Standards and technology (NIST) selected the Rijndael algorithm for AES because it offers a combination of security, performance, efficiency, ease of implementation, and flexibility. Specifically, Rijndael appears to be consistently a very good performer in both hardware and software across a wide range of computing environments regardless of its use in feedback or non- feedback modes. Its key setup time is excellent, and its key agility is good. The very low memory requirements of the Rijndael algorithm make it very well suited for restricted-space environments, in which it also demonstrates excellent performance. The Rijndael algorithm operations are among the easiest to defend against power and timing attacks. Additionally, it appears that some defense can be provided against such attacks without significantly impacting the algorithm's performance. Finally, the algorithm's internal round structure appears to have good potential to benefit from instruction-level parallelism. The AES specifies three key sizes: 128, 192 and 256 bits. 1.2 RSA-OAEP When the variant of the RSA key transport algorithm specified in PKCS #1 Version 1.5 [PKCS#1v1.5] is used for key management, it is vulnerable to adaptive chosen ciphertext attacks. This attack is described in [RSALAB] and [CRYPTO98]. The use of PKCS #1 Version 1.5 key transport in interactive applications is especially vulnerable, but countermeasures are described in [MMA]. Exploitation of this identified vulnerability, revealing the result of a particular RSA decryption, requires access to an oracle which will respond to Schaad, Housley 2 Use of the AES Algorithm in CMS February 2002 hundreds of thousands of ciphertexts, which are constructed adaptively in response to previously-received replies providing information on the successes or failures of attempted decryption operations. The attack appears significantly less feasible in store-and-forward environments, such as S/MIME. When PKCS #1 Version 1.5 key transport is applied as an intermediate encryption layer within an interactive request-response communications environment, exploitation could be more feasible. However, Secure Sockets Layer (SSL) [SSL] and Transport Layer Security (TLS) [TLS] protocol implementations could include countermeasures that detect and prevent Bleichenbacher's and other chosen-ciphertext attacks, without changing the way the RSA key transport algorithm is used. These countermeasures are performed within the protocol level. In the interest of long-term security assurance, it is prudent to adopt an improved cryptographic technique rather than embedding countermeasures in protocols. An updated version of PKCS #1 has been published: PKCS #1 Version 2.0 [PKCS#1v2.0]. This new document supersedes RFC 2313 [PKCS#1v1.5]. PKCS #1 Version 2.0 preserves support for the encryption padding format defined in PKCS #1 Version 1.5 [PKCS#1v1.5], and it also defines a new alternative. To resolve the adaptive chosen ciphertext vulnerability, the PKCS #1 Version 2.0 specifies and recommends use of Optimal Asymmetric Encryption Padding (OAEP) when RSA encryption is used to provide confidentiality, such as key transport. This document specifies the use of RSAES-OAEP key transport algorithm in the Cryptographic Message Syntax (CMS) [CMS]. CMS can be used in either a store-and-forward or an interactive request-response environment. CMS supports variety of architectures for certificate-based key management, particularly the one defined by the PKIX working group [PROFILE]. PKCS #1 Version 1.5 and PKCS #1 Version 2.0 require the same RSA public key information. Thus, a certified RSA public key may be used with either RSA key transport technique. 2 Enveloped-data Conventions The CMS enveloped-data content type consists of encrypted content and wrapped content-encryption keys for one or more recipients. The RSAES-OAEP key transport algorithm is used to wrap the content- encryption key for one recipient. The AES algorithm is used to encrypt the content. Compliant software MUST meet the requirements for constructing an enveloped-data content type stated in [CMS] Section 6, "Enveloped- data Content Type". An AES content-encryption key MUST be randomly generated for each instance of an enveloped-data content type. The content-encryption key (CEK) is used to encrypt the content. Schaad, Housley 3 Use of the AES Algorithm in CMS February 2002 AES can be used with the enveloped-data content type using any of the following key management techniques defined in [CMS] Section 6. 1) Key Transport: The AES CEK is uniquely wrapped for each recipient using the recipient's public RSA key and other values. Section 2.2 provides additional details. 2) Key Agreement: The AES CEK is uniquely wrapped for each recipient using a pairwise symmetric key-encryption key (KEK) generated using DH-ES [DH] using the originator's randomly generated private key, the recipient's public DH key, and other values. Section 2.3 provides additional details. 3) Previously Distributed Symmetric KEK: The AES CEK is wrapped using a previously distributed symmetric KEK (such as a Mail List Key). The methods by which the symmetric KEK is generated and distributed are beyond the scope of this document. Section 2.4 provides additional details. 4) Password Encryption: The AES CEK is wrapped using a KEK derived from a password or other shared secret. Section 2.5 provides additional details. 2.1 EnvelopedData Fields The enveloped-data content type is ASN.1 encoded using the EnvelopedData syntax. The fields of the EnvelopedData syntax MUST be populated as follows: The EnvelopedData version is determined based on a number of factors. See [CMS] section 6.1 for the algorithm to determine this value. The EnvelopedData originatorInfo field is not used for the RSAES-OAEP key transport algorithm. However, this field MAY be present to support recipients using other key management algorithms. The EnvelopedData recipientInfos CHOICE is dependent on the key management technique used. Section 2.2, 2.3, 2.4 and 2.5 provide additional information. The EnvelopedData encryptedContentInfo contentEncryptionAlgorithm field MUST specify a symmetric encryption algorithm. Implementations MUST support content encryption with AES, but implementations MAY support other algorithms as well. The EnvelopedData unprotectedAttrs MAY be present. 2.2 KeyTransRecipientInfo Fields The enveloped-data content type is ASN.1 encoded using the EnvelopedData syntax. The fields of the EnvelopedData syntax MUST be populated as follows: Schaad, Housley 4 Use of the AES Algorithm in CMS February 2002 The KeyTransRecipientInfo version MUST be either 0 or 2. If the RecipientIdentifier is the CHOICE issuerAndSerialNumber, then the version MUST be 0. If the RecipientIdentifier is subjectKeyIdentifier, then the version MUST be 2. The KeyTransRecipientInfo RecipientIdentifier provides two alternatives for specifying the recipient's certificate, and thereby the recipient's public key. The recipient's certificate MUST contain a RSA public key. The CEK is encrypted with the recipient's RSA public key. The issuerAndSerialNumber alternative identifies the recipient's certificate by the issuer's distinguished name and the certificate serial number; the subjectKeyIdentifier identifies the recipient's certificate by the X.509 subjectKeyIdentifier extension value. The KeyTransRecipientInfo keyEncryptionAlgorithm field specifies the RSAES-OAEP algorithm, and the associated parameters used to encrypt the CEK for the recipient. The key encryption process is described in [PKCS#1v2.0]. See section 4.2 of this document for the algorithm identifier and the parameter syntax. The KeyTransRecipientInfo encryptedKey is the result of encrypting the CEK with the recipient's RSA public key using the RSAES-OAEP algorithm. Note: When using a Triple-DES CEK, implementations MUST adjust the parity bits for each DES key comprising the Triple-DES key prior to RSAES-OAEP encryption. Note: The same key wrap algorithm is used for both Two-key Triple-DES and Three-key Triple-DES CEK keys. When a Two-key Triple-DES key is to be wrapped, a third DES key with the same value as the first DES key is created. Thus, all wrapped Triple-DES keys include three DES keys. 2.3 KeyAgreeRecipientInfo Fields This section describes the conventions for using ES-DH and AES with the CMS enveloped-data content type to support key agreement. When key agreement is used, then the RecipientInfo keyAgreeRecipientInfo CHOICE MUST be used. The KeyAgreeRecipient version MUST be 3. The EnvelopedData originatorInfo field MUST be the originatorKey alternative. The originatoryKey algorithm fields MUST contain the dh-public-number object identifier with absent parameters. The originatorKey publicKey MUST contain the originator's ephemeral public key. The EnvelopedData ukm MAY be present. The EnvelopedData keyEncrytionAlgorithm MUST be the id-alg-ESDH algorithm identifier [CMSALG]. Schaad, Housley 5 Use of the AES Algorithm in CMS February 2002 2.3.1 ES-DH/AES Key Derivation Generation of the AES KEK to be used with the AES -key wrap algorithm is done using the method described in [DH]. 2.3.1.1 Example 1 ZZ is the 20 bytes 00 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f 10 11 12 13 The key wrap algorithm is AES-128 wrap, so we need 128 bits (16 bytes) of keying material. No partyAInfo is used. Consequently, the input to SHA-1 is: 00 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f 10 11 12 13 ; ZZ 30 1b 30 11 06 09 60 86 48 01 65 03 04 01 05 ; AES-128 wrap OID 04 04 00 00 00 01 ; Counter a2 06 04 04 00 00 00 80 ; key length And the output is the 32 bytes: d6 d6 b0 94 c1 02 7a 7d e6 e3 11 72 94 a3 53 64 49 08 50 f9 Consenquently, K= d6 d6 b0 94 c1 02 7a 7d e6 e3 11 72 94 a3 53 64 2.3.1.2 Example 2 ZZ is the 20 bytes 00 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f 10 11 12 13 The key wrap algorithm is AES-256 key wrap, so we need 256 bits (32 bytes) of keying material. The partyAInfo used is the 64 bytes 01 23 45 67 89 ab cd ef fe dc ba 98 76 54 32 01 01 23 45 67 89 ab cd ef fe dc ba 98 76 54 32 01 01 23 45 67 89 ab cd ef fe dc ba 98 76 54 32 01 01 23 45 67 89 ab cd ef fe dc ba 98 76 54 32 01 Consequently, the input to first invocation of SHA-1 is: 00 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f 10 11 12 13 ; ZZ Schaad, Housley 6 Use of the AES Algorithm in CMS February 2002 30 5f 30 11 06 09 60 86 48 01 65 03 04 01 2c ; AES-256 wrap OID 04 04 00 00 00 01 ; Counter a0 42 04 40 01 23 45 67 89 ab cd ef fe dc ba 98 76 54 32 01 ; partyAInfo 01 23 45 67 89 ab cd ef fe dc ba 98 76 54 32 01 01 23 45 67 89 ab cd ef fe dc ba 98 76 54 32 01 01 23 45 67 89 ab cd ef fe dc ba 98 76 54 32 01 a2 06 04 04 00 00 01 00 ; key length And the output is the 20 bytes: 6f da b9 fa 67 09 30 3e 7e 2f 68 50 29 6f 28 fb 1b a6 4e 2a The input to second invocation of SHA-1 is: 00 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f 10 11 12 13 ; ZZ 30 5f 30 11 06 09 60 86 48 01 65 03 04 01 2c ; AES-256 wrap OID 04 04 00 00 00 02 ; Counter a0 42 04 40 01 23 45 67 89 ab cd ef fe dc ba 98 76 54 32 01 ; partyAInfo 01 23 45 67 89 ab cd ef fe dc ba 98 76 54 32 01 01 23 45 67 89 ab cd ef fe dc ba 98 76 54 32 01 01 23 45 67 89 ab cd ef fe dc ba 98 76 54 32 01 a2 06 04 04 00 00 01 00 ; key length And the output is the 20 bytes: 73 36 a5 ae 90 33 31 39 cb 3f 0e 90 cd d8 03 96 66 36 61 b0 Consequently, K = 6f da b9 fa 67 09 30 3e 7e 2f 68 50 29 6f 28 fb 1b a6 4e 2a 73 36 a5 ae 90 33 31 39 cb 3f 0e 90 2.3.2 AES CEK Wrap Process The AES key wrap algorithm encrypts one AES key in another AES key. The algorithm produces an output 64-bits longer than the input AES CEK, the additional bits are a checksum. The algorithm uses 6*n AES encryption/decryption operations where n is number of 64-bit blocks Schaad, Housley 7 Use of the AES Algorithm in CMS February 2002 in the AES CEK. Full details of the AES key wrap algorithm are available at [AES-WRAP]. NIST has assigned the following OIDs to define the AES key wrap algorithm. id-aes128-wrap OBJECT IDENTIFIER ::= { aes 5 } id-aes192-wrap OBJECT IDENTIFIER ::= { aes 25 } id-aes256-wrap OBJECT IDENTIFIER ::= { aes 45 } In all cases the parameters field MUST be absent. The OID gives the KEK key size, but does not make any statements as to the size of the wrapped AES CEK. Implementations MAY use different KEK and CEK sizes. Implements MUST support the CEK and the KEK having the same length. If different lengths are supported, the KEK MUST be of equal or greater length than the CEK. 2.4 KEKRecipientInfo Fields This section describes the conventions for using AES with the CMS enveloped-data content type to support previously distributed symmetric KEKs. When a previously distributed symmetric KEK is used to wrap the AES CEK, then the RecipientInfo KEKRecipientInfo CHOICE MUST be used. The methods used to generate and distribute the symmetric KEK are beyond the scope of this document. One possible method of distributing keys is documented in [SYMKEYDIST]. The KEKRecipientInfo fields MUST be populated as specified in [CMS] Section 6.2.3, KEKRecipientInfo Type. The KEKRecipientInfo keyEncryptionAlgorithm algorithm field MUST be one of the OIDs defined in section 2.3.2 indicating that the AES wrap function is used to wrap the AES CEK. The KEKRecipientInfo keyEncryptionAlgorithm parameters field MUST be absent. The KEKRecipientInfo encryptedKey field MUST include the AES CEK wrapped using the previously distributed symmetric KEK as input to the AES wrap function. 2.5 PasswordRecipientInfo Fields This section describes the conventions for using AES with the CMS enveloped-data content type to support password-based key management. When a password derived KEK is used to wrap the AES CEK, then the RecipientInfo PasswordRecipientInfo CHOICE MUST be used. The keyEncryptionAlgorithm algorithm field MUST be one of the OIDs defined in section 2.3.2 indicating the AES wrap function is used to wrap the AES CEK. The keyEncryptionAlgorithm parameters field MUST be absent. The encryptedKey field MUST be the result of the AES key wrap algorithm applied to the AES CEK value. Schaad, Housley 8 Use of the AES Algorithm in CMS February 2002 3 Encrypted-data Conventions The encrypted-data content type is ASN.1 encoded using the EncryptededData syntax. The fields of the EncryptedData syntax MUST be populated as follows: The EncryptedData version is determined based on a number of factors. See [CMS] section 9.1 for the algorithm to determine this value. The EncryptedData encryptedContentInfo contentEncryptionAlgorithm field MUST specify a symmetric encryption algorithm. Implementations MUST support encryption using AES, but implementations MAY support other algorithms as well. The EncryptedData unprotectedAttrs MAY be present. 4 Algorithm Identifiers and Parameters This section specified algorithm identifiers for the AES encryption algorithm and the RSAES-OAEP key transport algorithm. 4.1 AES Algorithm Identifiers and Parameters The AES algorithm is defined in [AES]. RSA #1 v1.5 [PKCS#1v1.5] MUST NOT be used to transport AES keys. RSAES-OAEP [PKCS#1v2.0] MAY be used to transport AES keys. AES is added to the set of symmetric content encryption algorithms defined in [CMSALG]. The AES content-encryption algorithm, in Cipher Block Chaining (CBC) mode, for the three different key sizes are identified by the following object identifiers: id-aes128-CBC OBJECT IDENTIFIER ::= { aes 2 } id-aes192-CBC OBJECT IDENTIFIER ::= { aes 22 } id-aes256-CBC OBJECT IDENTIFIER ::= { aes 42 } The AlgorithmIdentifier parameters field MUST be present, and the parameters field MUST contain a AES-IV: AES-IV ::= OCTET STRING (SIZE(16)) Content encryption algorithm identifiers are located in the EnvelopedData EncryptedContentInfo contentEncryptionAlgorithm and the EncryptedData EncryptedContentInfo contentEncryptionAlgorithm fields. Content encryption algorithms are used to encrypt the content located in the EnvelopedData EncryptedContentInfo encryptedContent and the EncryptedData EncryptedContentInfo encryptedContent fields. 4.2 RSAES-OAEP Algorithm Identifiers and Parameters The RSAES-OAEP key transport algorithm is the RSA encryption scheme defined in RFC 2437 [PKCS#1v2.0], where the message to be encrypted is the content-encryption key. Schaad, Housley 9 Use of the AES Algorithm in CMS February 2002 The RSA key is identified in a certificate using the rsaEncryption object identifier: pkcs-1 OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-1(1) } rsaEncryption OBJECT IDENTIFIER ::= { pkcs-1 1 } Note: This is the same algorithm identifier used by RSAES-PKCS1-v1_5. This means that the existence of an RSA key in a certificate cannot be used to infer that a recipient can decrypt an RSAES-OAEP encrypted content-encryption key. The object identifier for RSAES-OAEP is: id-RSAES-OAEP OBJECT IDENTIFIER ::= { pkcs-1 7 } The AlgorithmIdentifier parameters field MUST be present, and the parameters field MUST contain RSAES-OAEP-params. RSAES-OAEP-params have the following syntax: RSAES-OAEP-params ::= SEQUENCE { hashFunc [0] AlgorithmIdentifier DEFAULT sha1Identifier, maskGenFunc [1] AlgorithmIdentifier DEFAULT mgf1SHA1Identifier, pSourceFunc [2] AlgorithmIdentifier DEFAULT pSpecifiedEmptyIdentifier } sha1Identifier ::= AlgorithmIdentifier { id-sha1, NULL } mgf1SHA1Identifier ::= AlgorithmIdentifier { id-mgf1, sha1Identifier } pSpecifiedEmptyIdentifier ::= AlgorithmIdentifier { id-pSpecified, nullOctetString } id-sha1 OBJECT IDENTIFIER ::= { iso(1) identified-organization(3) oiw(14) secsig(3) algorithms(2) 26 } id-mgf1 OBJECT IDENTIFIER ::= { pkcs-1 8 } id-pSpecified OBJECT IDENTIFIER ::= { pkcs-1 9 } nullOctetString OCTET STRING (SIZE (0)) ::= { ''H } The fields of type RSAES-OAEP-params have the following meanings: hashFunc identifies the one-way hash function. Implementations MUST support SHA-1 [SHA1]. The SHA-1 algorithm identifier is comprised of the id-sha1 object identifier and a parameter of NULL. Implementations that perform key encryption MUST omit the hashFunc field when SHA-1 is used, indicating that the default algorithm was Schaad, Housley 10 Use of the AES Algorithm in CMS February 2002 used. Implementations that perform key decryption MUST recognize both the id-sha1 object identifier and an absent hashFunc field as an indication that SHA-1 was used. maskGenFunc identifies the mask generation function. Implementations MUST support MFG1 [PKCS#1v2.0]. MFG1 requires a one-way hash function, and it is identified in the parameter field of the MFG1 algorithm identifier. Implementations MUST support SHA-1 [SHA1]. The MFG1 algorithm identifier is comprised of the id-mgf1 object identifier and a parameter that contains the algorithm identifier of the one-way hash function employed with MFG1. The SHA-1 algorithm identifier is comprised of the id-sha1 object identifier and a parameter of NULL. Implementations that perform key encryption MUST omit the maskGenFunc field when MFG1 with SHA-1 is used, indicating that the default algorithm was used. Implementations that perform key decryption MUST recognize both the id-mgf1 and id-sha1 object identifiers as well as an absent maskGenFunc field as an indication that MFG1 with SHA-1 was used. pSourceFunc identifies the source (and possibly the value) of the encoding parameters, commonly called P. Implementations MUST represent P by an algorithm identifier, id-pSpecified, indicating that P is explicitly provided as an OCTET STRING in the parameters. The default value for P is an empty string. In this case, pHash in EME-OAEP contains the hash of a zero length string. Implementations MUST support a zero length P value. Implementations that perform key encryption MUST omit the pSourceFunc field when a zero length P value is used, indicating that the default value was used. Implementations that perform key decryption MUST recognize both the id-pSpecified object identifier and an absent pSourceFunc field as an indication that a zero length P value was used. 5 SMIMECapabilities Attribute Conventions An S/MIME client SHOULD announce the set of cryptographic functions it supports by using the S/MIME capabilities attribute. This attribute provides a partial list of object identifiers of cryptographic functions and MUST be signed by the client. The algorithm OIDs SHOULD be logically separated in functional categories and MUST be ordered with respect to their preference. RFC 2633 [MSG], Section 2.5.2 defines the SMIMECapabilities signed attribute (defined as a SEQUENCE of SMIMECapability SEQUENCEs) to be used to specify a partial list of algorithms that the software announcing the SMIMECapabilities can support. 5.1 RSAES-OEAP SMIMECapability Attribute When constructing a signedData object, compliant software MAY include the SMIMECapabilities signed attribute announcing that it supports the RSAES-OAEP algorithm. The SMIMECapability SEQUENCE representing RSAES-OAEP MUST include the id-RSAES-OAEP object identifier in the capabilityID field and MUST Schaad, Housley 11 Use of the AES Algorithm in CMS February 2002 include the RSAES-OAEP-Default-Identifier SEQUENCE in the parameters field. RSAES-OAEP-Default-Identifier ::= AlgorithmIdentifier { id-RSAES-OAEP, { sha1Identifier, mgf1SHA1Identifier, pSpecifiedEmptyIdentifier } } When all of the default settings are selected, the SMIMECapability SEQUENCE representing RSAES-OAEP MUST be DER-encoded as: 30 0D 06 09 2A 86 48 86 F7 0D 01 01 07 30 00 5.2 AES S/MIME Capability Attributes If an S/MIME client is required to support symmetric encryption with AES, the capabilities attribute MUST contain the AES object identifier specified above in the category of symmetric algorithms. The parameter associated with this object identifier MUST is AESSMimeCapability. AESSMimeCapabilty ::= NULL The encodings for the mandatory key sizes are: Key Size Capability 128 30 0D 06 09 60 86 48 01 65 03 04 01 02 30 00 196 30 0D 06 09 60 86 48 01 65 03 04 01 16 30 00 256 30 0D 06 09 60 86 48 01 65 03 04 01 2A 30 00 When a sending agent creates an encrypted message, it has to decide which type of encryption algorithm to use. In general the decision process involves information obtained from the capabilities lists included in messages received from the recipient, as well as other information such as private agreements, user preferences, legal restrictions, and so on. If users require AES for symmetric encryption, the S/MIME clients on both the sending and receiving side MUST support it, and it MUST be set in the user preferences. 6 Security Considerations If RSA-OAEP and RSA #1 v1.5 are both used to transport the same CEK, then an attacker can still use the Bleichenbacher attack against the RSA #1 v1.5 encrypted key. It is generally unadvisable to mix both RSA-OAEP and RSA #1 v1.5 in the same set of recipients. Implementations must protect the RSA private key and the CEK. Compromise of the RSA private key may result in the disclosure of all messages protected with that key. Compromise of the CEK may result in disclosure of the associated encrypted content. The generation of AES CEKs, RSA public/private key pairs, and MGF seeds rely on random numbers. The use of inadequate pseudo-random number generators (PRNGs) to generate these values can result in little or no security. An attacker may find it much easier to Schaad, Housley 12 Use of the AES Algorithm in CMS February 2002 reproduce the PRNG environment that produced the keys, searching the resulting small set of possibilities, rather than brute force searching the whole key space. The generation of quality random numbers is difficult. RFC 1750 [RANDOM] offers important guidance in this area. When wrapping a CEK with a KEK, the KEK MUST always be at least the same length as the CEK. An attacker will generally work at the weakest point in an encryption system. This would be the smaller of the two key sizes for a brute force attack. References AES National Institute of Standards. FIPS Pub 197: Advanced Encryption Standard (AES). 26 November 2001. AES-WRAP Schaad, J., R. Housley, "AES Key Wrap Algorithm", Draft-ietf-smime-aes-key-wrap-00.txt CMS Housley, R., Cryptographic Message Syntax. draft-ietf-smime-rfc2630bis-06.txt. CMSALG Housley, R., Cryptographic Message Syntax (CMS) Algorithms, draft-ietf-smime-cmsalg-07.txt. CRYPTO98 Bleichenbacher, D., "Chosen Ciphertext Attacks Against Protocols Based on the RSA Encryption Standard PKCS #1," in H. Krawczyk (editor), Advances in Cryptology - CRYPTO'98 Proceedings, Lecture Notes in Computer Science 1462 (1998), Springer-Verlag, pp. 1-12. DES National Institute of Standards and Technology. FIPS Pub 46: Data Encryption Standard. 15 January 1977. DH Rescorla, E., Diffie-Hellman Key Agreement Method, RFC 2631, June 1999. MUSTSHOULD Bradner, S., Key Words for Use in RFCs to Indicate Requirement Levels. BCP 14, RFC 2119. March 1997. MMA Rescorla, E., Preventing the Million Message Attack on CMS, RFC 3218, January 2002. MSG Ramsdell, B., Editor. S/MIME Version 3 Message Specification. RFC 2633. June 1999. PKCS#1v1.5 Kaliski, B. PKCS #1: RSA Encryption, Version 1.5. RFC 2313. March 1998. PKCS#1v2.0 Kaliski, B. PKCS #1: RSA Encryption, Version 2.0. RFC 2437. October 1998. Schaad, Housley 13 Use of the AES Algorithm in CMS February 2002 PROFILE Housley, R., W. Ford, W. Polk, and D. Solo. Internet X.509 Public Key Infrastructure: Certificate and CRL Profile. <draft-ietf-smime-new-part1.txt>. RANDOM Eastlake, D., S. Crocker, and J. Schiller. Randomness Recommendations for Security. RFC 1750. December 1994. RSALABS Bleichenbacher, D., B. Kaliski, and J. Staddon. Recent Results on PKCS #1: RSA Encryption Standard. RSA Laboratories' Bulletin No. 7, June 26, 1998. [At http://www.rsasecurity.com/rsalabs/bulletins] SHA1 National Institute of Standards and Technology. FIPS Pub 180-1: Secure Hash Standard. 17 April 1995. SSL Freier, A., P. Karlton, and P. Kocher. The SSL Protocol, Version 3.0. Netscape Communications. November 1996. [At http://www.netscape.com/eng/ssl3/draft302.txt] SYMKEYDIST Turner, S. CMS Symmetric Key Management and Distribution. RFC TDB. Date TBD. <draft-ietf-smime-symkeydist-06.txt> TLS Dierks, T. and C. Allen. The TLS Protocol Version 1.0. RFC 2246. January 1999. X.208-88 CCITT. Recommendation X.208: Specification of Abstract Syntax Notation One (ASN.1). 1988. X.209-88 CCITT. Recommendation X.209: Specification of Basic Encoding Rules for Abstract Syntax Notation One (ASN.1). 1988. X.509-88 CCITT. Recommendation X.509: The Directory - Authentication Framework. 1988. Acknowledgements This document is the result of contributions from many professionals. We appreciate the hard work of all members of the IETF S/MIME Working Group. We wish to extend a special thanks to Burt Kaliski. Author's Addresses Jim Schaad Soaring Hawk Consulting Email: jimsch@exmsft.com Russell Housley RSA Laboratories 918 Spring Knoll Drive Schaad, Housley 14 Use of the AES Algorithm in CMS February 2002 Herndon, VA 20170 USA Email: rhousley@rsasecurity.com Appendix A ASN.1 Module CMSAesRsaesOaep {iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) modules(0) aes-rsaes-oaep(19) } DEFINITIONS IMPLICIT TAGS ::= BEGIN -- EXPORTS ALL -- IMPORTS -- PKIX AlgorithmIdentifier FROM PKIXExplicit88 {iso(1) identified-organization(3) dod(6) internet(1) security(5) mechanisms(5) pkix(7) id-mod(0) id-pkix1-explicit(18)}; -- AES information object identifiers -- aes OBJECT IDENTIFIER ::= { joint-iso-itu-t(2) country(16) us(840) organization(1) gov(101) csor(3)_ nistAlgorithms(4) 1 } -- AES using CBC-chaining mode for key sizes of 128, 192, 256 id-aes128-CBC OBJECT IDENTIFIER ::= { aes 2 } id-aes192-CBC OBJECT IDENTIFIER ::= { aes 22 } id-aes256-CBC OBJECT IDENTIFIER ::= { aes 42 } -- AES-IV is a the parameter for all the above object identifiers. AES-IV ::= OCTET STRING (SIZE(16)) -- AES S/MIME Capabilty parameter for all the above object identifiers AESSMimeCapability ::= NULL -- Definitions for RSA-OAEP pkcs-1 OBJECT IDENTIFIER ::= {iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-1(1) } id-RSAES-OAEP OBJECT IDENTIFIER ::= { pkcs-1 7 } RSAES-OAEP-params ::= SEQUENCE { hashFunc [0] AlgorithmIdentifier DEFAULT sha1Identifier, maskGenFunc [1] AlgorithmIdentifier DEFAULT mgf1SHA1Identifier, pSourceFunc [2] AlgorithmIdentifier DEFAULT pSpecifiedEmptyIdentifier } sha1Identifier AlgorithmIdentifier ::= { id-sha1, NULL } Schaad, Housley 15 Use of the AES Algorithm in CMS February 2002 mgf1SHA1Identifier AlgorithmIdentifier ::= {id-mgf1, sha1Identifier } nullOctetString OCTET STRING (SIZE (0)) ::= { ''H } pSpecifiedEmptyIdentifier AlgorithmIdentifier ::= { id-pSpecified, nullOctetString } id-sha1 OBJECT IDENTIFIER ::= { iso(1) identified-organization(3) oiw(14) secsig(3) algorithms(2) 26 } id-mgf1 OBJECT IDENTIFIER ::= { pkcs-1 8 } id-pSpecified OBJECT IDENTIFIER ::= { pkcs-1 9 } rSAES-OAEP-Default-Identifier AlgorithmIdentifier ::= { id-RSAES-OAEP, { sha1Identifier, mgf1SHA1Identifier, pSpecifiedEmptyIdentifier } } END Schaad, Housley 16
Datatracker
draft-ietf-smime-aes-alg-04
Internet-Draft
