iOS is giving decryption error: RSAdecrypt wrong input (err -27)

Question

Diclaimer

In the first place this is a recap on recent development activities. I am writing this down for lost souls - as I could not find any post containing this error except:

RSA encryption/decryption implementing in Swift from Android Java code

The Problem

During decryption of a cyphertext with a given private key, iOS throws this error:

Optional(Swift.Unmanaged<__C.CFErrorRef>(_value: Error Domain=NSOSStatusErrorDomain Code=-50 "RSAdecrypt wrong input (err -27)" UserInfo={numberOfErrorsDeep=0, NSDescription=RSAdecrypt wrong input (err -27)}))

For the sake of readability and search engine optimization the same error is given in a style which is more easy to be read:

Optional(
    Swift.Unmanaged<__C.CFErrorRef>(
        _value: Error
            Domain=NSOSStatusErrorDomain
            Code=-50 "RSAdecrypt wrong input (err -27)"
            UserInfo={
                numberOfErrorsDeep=0,
                NSDescription=RSAdecrypt wrong input (err -27)
            }
    )
)

The Question

What are likely reasons for receiving this error?

However

My knowledge so far is given in one of the answers below ;-)

You should edit your text into a clear question, then add a separate answer to it (which I suppose is what you included as "the good news and the bad news", or perhaps is only the latter, not sure). — not2savvy, Feb 10 '23 at 17:13

d.braun1991 · Answer 1 · 2023-04-01T13:51:56.363

As mentioned by @not2savvy, I better answer as far as known here instead of 'within the question part' ... .. .

The good news

You probaply solved several problems already, such as:

Base64 decoding error
- -> In case your ciphertext/private key had to be transformed back to binary
Algorithm error
- -> Mismatch of PrivateKey in regards of the needed algorithm (e.g.: RSA)
Size error
- -> Mismatch of Ciphertext- and Keysize
Wrongkey error
- -> In case you generate keys, always using the same TAG ... ... Did you properly delete all other Keys using the same TAG from the keystore? If there are multiple, sometimes the wrong private is grabbed and decryption fails. Just as said in: https://developer.apple.com/documentation/security/certificate_key_and_trust_services/keys/generating_new_cryptographic_keys

Note

Be sure that you don’t generate multiple, identically tagged keys. ... .. .

... .. . you are not far from the solution ;-)

The bad news

You probaply have the wrong private key to decrypt the given cyphertext.
Something else is wrong with your key, which I do know nothing about.