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Editing: PROTOCOL.krl
This describes the key/certificate revocation list format for OpenSSH. 1. Overall format The KRL consists of a header and zero or more sections. The header is: #define KRL_MAGIC 0x5353484b524c0a00ULL /* "SSHKRL\n\0" */ #define KRL_FORMAT_VERSION 1 uint64 KRL_MAGIC uint32 KRL_FORMAT_VERSION uint64 krl_version uint64 generated_date uint64 flags string reserved string comment Where "krl_version" is a version number that increases each time the KRL is modified, "generated_date" is the time in seconds since 1970-01-01 00:00:00 UTC that the KRL was generated, "comment" is an optional comment and "reserved" an extension field whose contents are currently ignored. No "flags" are currently defined. Following the header are zero or more sections, each consisting of: byte section_type string section_data Where "section_type" indicates the type of the "section_data". An exception to this is the KRL_SECTION_SIGNATURE section, that has a slightly different format (see below). The available section types are: #define KRL_SECTION_CERTIFICATES 1 #define KRL_SECTION_EXPLICIT_KEY 2 #define KRL_SECTION_FINGERPRINT_SHA1 3 #define KRL_SECTION_SIGNATURE 4 #define KRL_SECTION_FINGERPRINT_SHA256 5 #define KRL_SECTION_EXTENSION 255 2. Certificate section These sections use type KRL_SECTION_CERTIFICATES to revoke certificates by serial number or key ID. The consist of the CA key that issued the certificates to be revoked and a reserved field whose contents is currently ignored. string ca_key string reserved Where "ca_key" is the standard SSH wire serialisation of the CA's public key. Alternately, "ca_key" may be an empty string to indicate the certificate section applies to all CAs (this is most useful when revoking key IDs). Followed by one or more sections: byte cert_section_type string cert_section_data The certificate section types are: #define KRL_SECTION_CERT_SERIAL_LIST 0x20 #define KRL_SECTION_CERT_SERIAL_RANGE 0x21 #define KRL_SECTION_CERT_SERIAL_BITMAP 0x22 #define KRL_SECTION_CERT_KEY_ID 0x23 #define KRL_SECTION_CERT_EXTENSION 0x39 2.1 Certificate serial list section This section is identified as KRL_SECTION_CERT_SERIAL_LIST. It revokes certificates by listing their serial numbers. The cert_section_data in this case contains: uint64 revoked_cert_serial uint64 ... This section may appear multiple times. 2.2. Certificate serial range section These sections use type KRL_SECTION_CERT_SERIAL_RANGE and hold a range of serial numbers of certificates: uint64 serial_min uint64 serial_max All certificates in the range serial_min <= serial <= serial_max are revoked. This section may appear multiple times. 2.3. Certificate serial bitmap section Bitmap sections use type KRL_SECTION_CERT_SERIAL_BITMAP and revoke keys by listing their serial number in a bitmap. uint64 serial_offset mpint revoked_keys_bitmap A bit set at index N in the bitmap corresponds to revocation of a keys with serial number (serial_offset + N). This section may appear multiple times. 2.4. Revoked key ID sections KRL_SECTION_CERT_KEY_ID sections revoke particular certificate "key ID" strings. This may be useful in revoking all certificates associated with a particular identity, e.g. a host or a user. string key_id[0] ... This section must contain at least one "key_id". This section may appear multiple times. 2.5. Certificate Extension subsections This subsection type provides a generic extension mechanism to the certificates KRL section that may be used to provide optional or critical data. Extensions are stored in subsections of type KRL_SECTION_CERT_EXTENSION with the following contents: string extension_name boolean is_critical string extension_contents. Where "extension_name" describes the type of extension. It is recommended that user extensions follow "cert-name@domain.org" naming. The "is_critical" indicates whether this extension is mandatory or optional. If true, then any unsupported extension encountered should result in KRL parsing failure. If false, then it may be safely be ignored. The "extension_contents" contains the body of the extension. 3. Explicit key sections These sections, identified as KRL_SECTION_EXPLICIT_KEY, revoke keys (not certificates). They are less space efficient than serial numbers, but are able to revoke plain keys. string public_key_blob[0] .... This section must contain at least one "public_key_blob". The blob must be a raw key (i.e. not a certificate). This section may appear multiple times. 4. SHA1/SHA256 fingerprint sections These sections, identified as KRL_SECTION_FINGERPRINT_SHA1 and KRL_SECTION_FINGERPRINT_SHA256, revoke plain keys (i.e. not certificates) by listing their hashes: string public_key_hash[0] .... This section must contain at least one "public_key_hash". The hash blob is obtained by taking the SHA1 or SHA256 hash of the public key blob. Hashes in this section must appear in numeric order, treating each hash as a big-endian integer. This section may appear multiple times. 5. Extension sections This section type provides a generic extension mechanism to the KRL format that may be used to provide optional or critical data. Extensions are recorded in sections of type KRL_SECTION_EXTENSION with the following contents: string extension_name boolean is_critical string extension_contents. Where "extension_name" describes the type of extension. It is recommended that user extensions follow "name@domain.org" naming. The "is_critical" indicates whether this extension is mandatory or optional. If true, then any unsupported extension encountered should result in KRL parsing failure. If false, then it may be safely be ignored. The "extension_contents" contains the body of the extension. 6. KRL signature sections Note: KRL signatures are not supported by OpenSSH. OpenSSH >= 9.4 will refuse to load KRLs that contain signatures. We recommend the use of SSHSIG (`ssh-keygen -Y sign ...`) style signatures for KRLs instead. The KRL_SECTION_SIGNATURE section serves a different purpose to the preceding ones: to provide cryptographic authentication of a KRL that is retrieved over a channel that does not provide integrity protection. Its format is slightly different to the previously-described sections: in order to simplify the signature generation, it includes as a "body" two string components instead of one. byte KRL_SECTION_SIGNATURE string signature_key string signature The signature is calculated over the entire KRL from the KRL_MAGIC to this subsection's "signature_key", including both and using the signature generation rules appropriate for the type of "signature_key". This section must appear last in the KRL. If multiple signature sections appear, they must appear consecutively at the end of the KRL file. Implementations that retrieve KRLs over untrusted channels must verify signatures. Signature sections are optional for KRLs distributed by trusted means. $OpenBSD: PROTOCOL.krl,v 1.7 2023/07/17 04:01:10 djm Exp $
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