What makes an Ethereum keypair valid

Using a Library

Libraries like web3.js and ethers have isAddress().

Examples:

• ethers.utils.isAddress('0x8ba1f109551bd432803012645ac136ddd64dba72'); // true

• web3.utils.isAddress('blah'); // false

The following is an answer from 2016.

Regular Address

EIP 55 added a "capitals-based checksum" which was implemented by Geth by May 2016. Here's Javascript code from Geth:

/**
 * Checks if the given string is an address
 *
 * @method isAddress
 * @param {String} address the given HEX adress
 * @return {Boolean}
*/
var isAddress = function (address) {
    if (!/^(0x)?[0-9a-f]{40}$/i.test(address)) {
        // check if it has the basic requirements of an address
        return false;
    } else if (/^(0x)?[0-9a-f]{40}$/.test(address) || /^(0x)?[0-9A-F]{40}$/.test(address)) {
        // If it's all small caps or all all caps, return true
        return true;
    } else {
        // Otherwise check each case
        return isChecksumAddress(address);
    }
};

/**
 * Checks if the given string is a checksummed address
 *
 * @method isChecksumAddress
 * @param {String} address the given HEX adress
 * @return {Boolean}
*/
var isChecksumAddress = function (address) {
    // Check each case
    address = address.replace('0x','');
    var addressHash = sha3(address.toLowerCase());
    for (var i = 0; i < 40; i++ ) {
        // the nth letter should be uppercase if the nth digit of casemap is 1
        if ((parseInt(addressHash[i], 16) > 7 && address[i].toUpperCase() !== address[i]) || (parseInt(addressHash[i], 16) <= 7 && address[i].toLowerCase() !== address[i])) {
            return false;
        }
    }
    return true;
};

ICAP Address

ICAP has a checksum which can be verified. You can review Geth's icap.go and here's a snippet from it:

// https://en.wikipedia.org/wiki/International_Bank_Account_Number#Validating_the_IBAN
func validCheckSum(s string) error {
    s = join(s[4:], s[:4])
    expanded, err := iso13616Expand(s)
    if err != nil {
        return err
    }
    checkSumNum, _ := new(big.Int).SetString(expanded, 10)
    if checkSumNum.Mod(checkSumNum, Big97).Cmp(Big1) != 0 {
        return ICAPChecksumError
    }
    return nil
}

The problem was the wrongly calculated address.

signerPrivKey = eth_keys.keys.PrivateKey(priv_key.to_bytes(32, "big"))
signerPubKey = signerPrivKey.public_key
print("address: ", signerPubKey.to_checksum_address())
#address:  0xE144236EFb7932bDaE617eBAAFD24aF962152c77