What is ECI on 3DS protocol?

Electronic Commerce Indicator (ECI) is a value returned by Directory Servers (namely Visa, MasterCard, JCB, and American Express) indicating the outcome of authentication attempted on transactions enforced by 3DS.

Possible value returned by Visa ↗, American Express, and JCB and its interpretation:

• ECI 05: 3DS authentication was successful; transactions are secured by 3DS.
• ECI 06: authentication was attempted but was not or could not be completed; possible reasons being either the card or its Issuing Bank has yet to participate in 3DS.
• ECI 07: 3DS authentication is either failed or could not be attempted; possible reasons being both card and Issuing Bank are not secured by 3DS, technical errors, or improper configuration.

Possible value returned by MasterCard ↗ and its interpretation:

• ECI 02: 3DS authentication is successful; both card and Issuing Bank are secured by 3DS.
• ECI 01: 3DS authentication was attempted but was not or could not be completed; possible reasons being either the card or its Issuing Bank has yet to participate in 3DS, or cardholder ran out of time to authorize.
• ECI 00: 3DS authentication is either failed or could not be attempted; possible reasons being both card and Issuing Bank are not secured by 3DS, technical errors, or improper configuration.