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// Package fingerprint provides a helper to create fingerprint string from certificate
package fingerprint
import (
"crypto"
"crypto/x509"
"errors"
"fmt"
)
var (
errHashUnavailable = errors.New("fingerprint: hash algorithm is not linked into the binary")
errInvalidFingerprintLength = errors.New("fingerprint: invalid fingerprint length")
)
// Fingerprint creates a fingerprint for a certificate using the specified hash algorithm
func Fingerprint(cert *x509.Certificate, algo crypto.Hash) (string, error) {
if !algo.Available() {
return "", errHashUnavailable
}
h := algo.New()
for i := 0; i < len(cert.Raw); {
n, _ := h.Write(cert.Raw[i:])
// Hash.Writer is specified to be never returning an error.
// https://golang.org/pkg/hash/#Hash
i += n
}
digest := []byte(fmt.Sprintf("%x", h.Sum(nil)))
digestlen := len(digest)
if digestlen == 0 {
return "", nil
}
if digestlen%2 != 0 {
return "", errInvalidFingerprintLength
}
res := make([]byte, digestlen>>1+digestlen-1)
pos := 0
for i, c := range digest {
res[pos] = c
pos++
if (i)%2 != 0 && i < digestlen-1 {
res[pos] = byte(':')
pos++
}
}
return string(res), nil
}
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