Firmware Key and Signature Formats

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This page describes the key and signature formats understood by OFW. The Firmware Security page describes how these are used.

Key

key01: data\n
 3 2 2  N  1

So that's:

• The literal string "key"
• The two digit version number ("01" for now)
• A colon
• A space
• The key data
• A newline

The key data is a hexadecimal-encoded octet string. The octet string is the ASN.1 DER encoding of an RSA public key given by Appendix A.1.1 of RSA PKCS #1, version 2.1.

Future versions of the key format might allow different algorithms.

Signature

Version 1

sig01: hashname keyid data\n
 3 2 2    6    1  64 1  N  1

So that's:

• The literal string "sig"
• The two digit version number ("01" for now)
• A colon
• A space
• A six character name for the hash function used by this signature
  • "sha256" indicates that this is an RSASSA-PSS signature using SHA256 as the hash and MGF1-SHA256 as the mask function.
  • "rmd160" indicates that this is an RSASSA-PKCS1-v1_5 signature using RIPEMD-160 as the hash function.
• A space
• The 64 character key ID, which are the trailing 64 characters of the "key data" in the key format above. (for the immediate future you can ignore this in the firmware, and just use a single key for each task.)
  • This includes the exponent and the least significant bytes of the modulus
• A space
• The signature data as a hexadecimal-encoded string. The encoded data is the octet string given by section 8.1.1/8.2.1 of RSA PKCS #1, version 2.1.
• A newline

Version 2

Version 2 signatures are (a) delegated, (b) tied to a particular serial number, and (c) may expire. The format is:

 sig02: hashname_1 key-or-keyid_1 expiration_1 data_1 hashname_2 ...\n
  3 2 2    6      1     N/64     1   16       1  M   1 ...           1

The "hashname key expiration data" section is repeated as many times as there are signatures in the chain. We will call each repetition an 'sg' below. So the contents of a sig02 are:

• The literal string "sig"
• The two digit version number ("02")
• A colon
• A space

At least one sg, consisting of:

• A six character name for the hash function used by this signature
  • "sha256" indicates that this is an RSASSA-PSS signature using SHA256 as the hash and MGF1-SHA256 as the mask function.
  • "rmd160" indicates that this is an RSASSA-PKCS1-v1_5 signature using RIPEMD-160 as the hash function.
• A space
• At least 64 characters of key data in the key01 format. The characters must be a suffix of the full key data; if more than 64 characters are present they are assumed to be the full key.
  • 64 characters is sufficient to include the exponent and the least significant bytes of the modulus
  • it is recommended that only keyid_1 be abbreviated.
• A space
• An ISO 8601 UTC expiration time in basic format (no dashes or colons) and no fractional seconds. (eg: "20070816T173500Z"). After this time, the sg must no longer be considered valid.
  • The string "00000000T000000Z" indicates that the given sg never expires.
• The signature data as a hexadecimal-encoded string. The encoded data is the octet string given by section 8.1.1/8.2.1 of RSA PKCS #1, version 2.1. The data to be signed is described below.
• A space (if this sg confers a delegation) or a newline (terminating the signature chain).

The sig02 line validates a byte string called the 'certified data'. For the final sg in the line, the 'data' field is a signature of the string:

  <serial number>:<expiration time>:<certified data>

using the keypair specified in the sg. For all other sgs, the 'data' field is a signature of the string:

  <serial number>:<expiration time>:<next public key in key01 format>

where key_1 is used to sign a string containing key_2, key_2 signs a string containing key_3, etc. The key in the signed string must never be abbreviated. Each delegation, and the final signature, is bound to a specific laptop serial number (which should be the same if the signature is to be meaningful). Use the sig01 format if you want an unbound signature.

A sig02 with delegation is formed by concatenating the signature of specific certified data onto a prefix of sgs which confer authority. A sig02 without delegation still adds an expiration time and a serial-number binding to the equivalent sig01-format signature.

Validating a file containing sig01 and sig02 signatures starts by scanning the file for sig01/sig02 signatures with an initial 'keyid' matching a trusted keypair. For a sig02, we then validate each sg in turn until we reach the final sg, which should validate against the certified data.

Antitheft/Activation Lease

act01: serial-number d expiration sig01: sha256 keyid data\n
 3 2 2      11      111     16   1 3 2 2    6  1  64 1  N  1
act01: serial-number d expiration sig02: sha256 keyid expiration data ...\n
 3 2 2      11      111     16   1 3 2 2    6  1 64/N 1  16     1 M      1

An activation lease begins with:

• The literal string 'act'
• The two digit version number ("01" for now)
• A colon
• A space
• The 11-character ASCII serial number of the machine this lease is for
• A space
• A one-character disposition
• A space
• The 16-character ISO 8601 UTC expiration time in basic format (no dashes or colons) and no fractional seconds. (eg: "20070816T173500Z")
  • should consist of the string "00000000T000000Z" if not present or not applicable.

It is then followed by a signature, in the sig01 or sig02 format above and with hashname "sha256", of the string:

<serial-number>:<uuid>:<disposition>:<expiration time>

where the expiration time in the string is identical in format and content to the signature expiration time.

For example:

SHF725001A0:414737D8-2312-9241-9C7B-9886CB74403C:K:20080819T052946Z

The disposition is ignored by firmware. Current userland behavior corresponds to disposition 'K'; we might support alternative dispositions in the future.

Developer key

dev01: serial-number d expiration sig01: sha256 keyid data\n
 3 2 2      11      111     16   1 3 2 2   6   1  64 1  N  1
dev01: serial-number d expiration sig02: sha256 keyid expiration data ...\n
 3 2 2      11      111     16   1 3 2 2   6   1 64/N 1 16      1 M      1

An developer key begins with:

• The literal string 'dev'
• The two digit version number ("01" for now)
• A colon
• A space
• The 11-character ASCII serial number of the machine this developer key is for
• A space
• The 1-character disposition
• A space
• The 16-character string '00000000T000000Z'
• A space

It is then followed by a signature, in the sig01 or sig02 format above and with hashname "sha256", of the string:

<serial-number>:<uuid>:<disposition>:00000000T000000Z

For example:

SHF725001A0:414737D8-2312-9241-9C7B-9886CB74403C:A:00000000T000000Z

Note that the payload is identical to that of an activation lease, although developer keys never expire. The verification key used also differs.

At the moment, developer keys always have disposition 'A'.

Resources

• PKCS #1, v2.1 (wikipedia)
• ASN.1 DER (wikipedia)