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|
# frozen_string_literal: true
module SSHData
class Signature
PEM_TYPE = "SSH SIGNATURE"
SIGNATURE_PREAMBLE = "SSHSIG"
MIN_SUPPORTED_VERSION = 1
MAX_SUPPORTED_VERSION = 1
# Spec: no SHA1 or SHA384. In practice, OpenSSH is always going to use SHA512.
# Note the actual signing / verify primitive may use a different hash algorithm.
# https://github.com/openssh/openssh-portable/blob/b7ffbb17e37f59249c31f1ff59d6c5d80888f689/PROTOCOL.sshsig#L67
SUPPORTED_HASH_ALGORITHMS = {
"sha256" => OpenSSL::Digest::SHA256,
"sha512" => OpenSSL::Digest::SHA512,
}
PERMITTED_RSA_SIGNATURE_ALGORITHMS = [
PublicKey::ALGO_RSA_SHA2_256,
PublicKey::ALGO_RSA_SHA2_512,
]
attr_reader :sigversion, :namespace, :signature, :reserved, :hash_algorithm
# Parses a PEM armored SSH signature.
# pem - A PEM encoded SSH signature.
#
# Returns a Signature instance.
def self.parse_pem(pem)
pem_type = Encoding.pem_type(pem)
if pem_type != PEM_TYPE
raise DecodeError, "Mismatched PEM type. Expecting '#{PEM_TYPE}', actually '#{pem_type}'."
end
blob = Encoding.decode_pem(pem, pem_type)
self.parse_blob(blob)
end
def self.parse_blob(blob)
data, read = Encoding.decode_openssh_signature(blob)
if read != blob.bytesize
raise DecodeError, "unexpected trailing data"
end
new(**data)
end
def initialize(sigversion:, publickey:, namespace:, reserved:, hash_algorithm:, signature:)
if sigversion > MAX_SUPPORTED_VERSION || sigversion < MIN_SUPPORTED_VERSION
raise UnsupportedError, "Signature version is not supported"
end
unless SUPPORTED_HASH_ALGORITHMS.has_key?(hash_algorithm)
raise UnsupportedError, "Hash algorithm #{hash_algorithm} is not supported."
end
# Spec: empty namespaces are not permitted.
# https://github.com/openssh/openssh-portable/blob/b7ffbb17e37f59249c31f1ff59d6c5d80888f689/PROTOCOL.sshsig#L57
raise UnsupportedError, "A namespace is required." if namespace.empty?
# Spec: ignore 'reserved', don't need to validate that it is empty.
@sigversion = sigversion
@publickey = publickey
@namespace = namespace
@reserved = reserved
@hash_algorithm = hash_algorithm
@signature = signature
end
def verify(signed_data, **opts)
signing_key = public_key
# Unwrap the signing key if this signature was created from a certificate.
key = signing_key.is_a?(Certificate) ? signing_key.public_key : signing_key
digest_algorithm = SUPPORTED_HASH_ALGORITHMS[@hash_algorithm]
if key.is_a?(PublicKey::RSA)
sig_algo, * = Encoding.decode_signature(@signature)
# Spec: If the signature is an RSA signature, the legacy 'ssh-rsa'
# identifer is not permitted.
# https://github.com/openssh/openssh-portable/blob/b7ffbb17e37f59249c31f1ff59d6c5d80888f689/PROTOCOL.sshsig#L72
unless PERMITTED_RSA_SIGNATURE_ALGORITHMS.include?(sig_algo)
raise UnsupportedError, "RSA signature #{sig_algo} is not supported."
end
end
message_digest = digest_algorithm.digest(signed_data)
blob =
SIGNATURE_PREAMBLE +
Encoding.encode_string(@namespace) +
Encoding.encode_string(@reserved || "") +
Encoding.encode_string(@hash_algorithm) +
Encoding.encode_string(message_digest)
if key.class.include?(::SSHData::PublicKey::SecurityKey)
key.verify(blob, @signature, **opts)
else
key.verify(blob, @signature)
end
end
# Gets the public key from the signature.
# If the signature was created from a certificate, this will be an
# SSHData::Certificate. Otherwise, this will be a PublicKey algorithm.
def public_key
@data_public_key ||= load_public_key
end
private def load_public_key
public_key_algorithm, _ = Encoding.decode_string(@publickey)
if PublicKey::ALGOS.include?(public_key_algorithm)
PublicKey.parse_rfc4253(@publickey)
elsif Certificate::ALGOS.include?(public_key_algorithm)
Certificate.parse_rfc4253(@publickey)
else
raise UnsupportedError, "Public key algorithm #{public_key_algorithm} is not supported."
end
end
end
end
|
