Issue
I have noticed that all the DKIM public keys generated always end with the string 'IDAQAB'. Any reason to it or are there cases where DKIM public keys will not end with the same string all the time ?.
Solution
DKIM public keys are encoded in the binary DER format and shared as Base64 in the DNS. RSA public keys consist of a modulus and an exponent. The exponent is typically 65537, which is 01 00 01
in hexadecimal. DER prefixes this value with 02
for the integer type and 03
for the length of the exponent in bytes. The Base64 encoding of 02 03 01 00 01
is IDAQAB
(at the right offset).
Before the modulus, which is unique for each RSA public key, there are nested length prefixes and an object identifier. This information is identical for RSA keys of the same length, which is why you find many DKIM public keys which also share the same prefix, such as MIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEA
.
Answered By - Kaspar Etter Answer Checked By - Katrina (PHPFixing Volunteer)
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