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Utility functions for implementing Proof Key for Code Exchange (PKCE) by OAuth
Public Clients

See RFC7636.
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    Generates a 'code_verifier' as described in section 4.1 of RFC 7636.

    This is a 'high-entropy cryptographic random string' that will be
    impractical for an attacker to guess.

    Args:
        n_bytes: integer between 31 and 96, inclusive. default: 64
            number of bytes of entropy to include in verifier.

    Returns:
        Bytestring, representing urlsafe base64-encoded random data.
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    Creates a 'code_challenge' as described in section 4.2 of RFC 7636
    by taking the sha256 hash of the verifier and then urlsafe
    base64-encoding it.

    Args:
        verifier: bytestring, representing a code_verifier as generated by
            code_verifier().

    Returns:
        Bytestring, representing a urlsafe base64-encoded sha256 hash digest,
            without '=' padding.
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