SPRUJ28E November 2021 – September 2024 AM68 , AM68A , TDA4AL-Q1 , TDA4VE-Q1 , TDA4VL-Q1
The X.509 certificate is described in RFC5280. Section 4.1 of the specification describes the format.
The X.509 fields relevant to the public boot (taken from RFC5280) are shown below.
Certificate ::= SEQUENCE {
tbsCertificate TBSCertificate,
signatureAlgorithm AlgorithmIdentifier,
signatureValue BIT STRING }
TBSCertificate ::= SEQUENCE {
version [0] EXPLICIT Version DEFAULT v1,
serialNumber CertificateSerialNumber,
signature AlgorithmIdentifier,
issuer Name,
validity Validity,
subject Name,
subjectPublicKeyInfo SubjectPublicKeyInfo,
issuerUniqueID [1] IMPLICIT UniqueIdentifier OPTIONAL,
-- If present, version MUST be v2 or v3
subjectUniqueID [2] IMPLICIT UniqueIdentifier OPTIONAL,
-- If present, version MUST be v2 or v3
extensions [3] EXPLICIT Extensions OPTIONAL
-- If present, version MUST be v3
}
Extensions ::= SEQUENCE SIZE (1..MAX) OF Extension
Extension ::= SEQUENCE {
extnID OBJECT IDENTIFIER,
critical BOOLEAN DEFAULT FALSE,
extnValue OCTET STRING
-- contains the DER encoding of an ASN.1 value
-- corresponding to the extension type identified
-- by extnID
}
In general, an X.509 certificate contains a public key which has been signed by a private key. The public ROM code does not directly use the keys. In non-secure devices, the public key value is in general a don’t care condition. The exception is certificates containing a degenerate RSA public key. GP devices with a degenerate RSA key allow for integrity checking of most (but not all) of the certificate.
The public MCU ROM only needs to extract some of information from the X.509 formatted structure:
The total size of the X.509 Certificate is determined by reading the length of the sequence containing the certificate. The length of the image is determined by parsing the certificate to find the extension field which holds the image length.
The ROM defines several extensions that are used only by TI for boot. These are placed in the extensions field of the TBS certificate.