Terms and Concepts - AWS Certificate Manager Private Certificate Authority

Terms and Concepts

The following terms and concepts can help you as you work with AWS Certificate Manager Private Certificate Authority (ACM Private CA).

Asymmetric Key Cryptography

Asymmetric cryptography uses different but mathematically related keys to encrypt and decrypt content. One of the keys is public and is made available in an X.509 v3 certificate. The other key is private and is stored securely. The X.509 certificate binds the identity of a user, computer, or other resource (the certificate subject) to the public key.

Private CA certificates and certificates issued by a private CA are X.509 SSL/TLS certificates. They bind the identity of a user, service, application, computer, or other device with the public key that's embedded in the certificate. The associated private key is securely stored by AWS.

Basic Constraints Path Length

The basic constraints path length is a certificate extension for CA certificates that defines the maximum CA Depth of the CA hierarchy that exists under a CA. For example, a CA with a path length constraint of zero cannot have any subordinate CAs. A CA with a path length constraint of one may have up to one level of subordinate CAs underneath it. RFC 5280 defines this as, “the maximum number of non-self-issued intermediate certificates that may follow this certificate in a valid certification path."

Certificate Authority

A certificate authority (CA) issues and if necessary revokes digital certificates. The most common type of certificate is based on the ISO X.509 standard. An X.509 certificate affirms the identity of the certificate subject and binds that identity to a public key. The subject can be a user, an application, a computer, or other device. The CA signs a certificate by hashing the contents and then encrypting the hash with the private key related to the public key in the certificate. A client application such as a web browser that needs to affirm the identity of a subject uses the public key to decrypt the certificate signature. It then hashes the certificate contents and compares the hashed value to the decrypted signature to determine whether they match. For information about key pairs, see Asymmetric Key Cryptography. For information about certificate signing, see Certificate Signature.

You can use ACM Private CA to create a private CA and use the private CA to issue certificates. Your private CA issues only private SSL/TLS certificates for use within your organization. For more information, see Private Certificate. Your private CA also requires a certificate before you can use it. For more information, see Certificate Authority Certificate.

Certificate Authority Certificate

A certificate authority (CA) certificate affirms the identity of the CA and binds it to the public key that is contained in the certificate.

You can use ACM Private CA to create a private root CA or a private subordinate CA, each backed by a CA certificate. Subordinate CA certificates are signed by another CA certificate higher in a chain of trust. But in the case of a root CA, the certificate is self-signed. You can also establish an external root authority (hosted on premises, for example). You can then use your root authority to sign a subordinate root CA certificate hosted by ACM Private CA.

The following example shows the typical fields contained in an ACM Private CA X.509 CA certificate. Note that for a CA certificate, the CA: value in the Basic Constraints field is set to TRUE.

Certificate: Data: Version: 3 (0x2) Serial Number: 4121 (0x1019) Signature Algorithm: sha256WithRSAEncryption Issuer: C=US, ST=Washington, L=Seattle, O=Example Company Root CA, OU=Corp, CN=www.example.com/emailAddress=corp@www.example.com Validity Not Before: Feb 26 20:27:56 2018 GMT Not After : Feb 24 20:27:56 2028 GMT Subject: C=US, ST=WA, L=Seattle, O=Examples Company Subordinate CA, OU=Corporate Office, CN=www.example.com Subject Public Key Info: Public Key Algorithm: rsaEncryption Public-Key: (2048 bit) Modulus: 00:c0: ... a3:4a:51 Exponent: 65537 (0x10001) X509v3 extensions: X509v3 Subject Key Identifier: F8:84:EE:37:21:F2:5E:0B:6C:40:C2:9D:C6:FE:7E:49:53:67:34:D9 X509v3 Authority Key Identifier: keyid:0D:CE:76:F2:E3:3B:93:2D:36:05:41:41:16:36:C8:82:BC:CB:F8:A0 X509v3 Basic Constraints: critical CA:TRUE X509v3 Key Usage: critical Digital Signature, CRL Sign Signature Algorithm: sha256WithRSAEncryption 6:bb:94: ... 80:d8

CA Depth

The number of certificates in a CA hierarchy starts with the root CA and traverses to subordinate CAs. The root counts as one and an additional certificate is counted for each step from a parent CA to a child at each level. For example, a trust chain with only a root CA and an end-entity certificate issued directly from the root has a depth (path length) of two. Each subordinate CA in the trust chain increases the depth by one. A root CA with one subordinate CA and an end-entity certificate has a depth of three.

CA Certificate

Identifies a certificate authority. CA certificates must include a special extensions known as a basic constraint flag. This flag must be set to true to allow the certificate to be used to issue certificates.

Certificate Signature

A digital signature is an encrypted hash over a certificate. A signature is used to affirm the integrity of the certificate data. Your private CA creates a signature by using a cryptographic hash function such as SHA256 over the variable-sized certificate content. This hash function produces an effectively unforgeable fixed-size data string. This string is called a hash. The CA then encrypts the hash value with its private key and concatenates the encrypted hash with the certificate.

To validate a signed certificate, a client application uses the CA public key to decrypt the signature. The client then uses the same signing algorithm that the CA used to compute a hash over the rest of the certificate. Note that the signing algorithm used by the CA is listed in the certificate. If the computed hash value is the same as the decrypted hash value, the certificate has not been tampered with.

Certificate Path

A client that relies on a certificate validates that a path exists from the end-entity certificate to a trusted root. The client checks that each certificate along the path is valid (not revoked). It also checks that the certificate has not expired, has integrity (has not been tampered with or modified), and that constraints in the certificate are enforced.

Domain Name System

The Domain Name System (DNS) is a hierarchical distributed naming system for computers and other resources connected to the internet or a private network. DNS is primarily used to translate textual domain names, such as aws.amazon.com, into numerical IP (internet protocol) addresses of the form

Domain Names

A domain name is a text string such as www.example.com that can be translated by the Domain Name System (DNS) into an IP address. Computer networks, including the internet, use IP addresses rather than text names. A domain name consists of distinct labels separated by periods.


The rightmost label is called the top-level domain (TLD). Common examples include .com, .net, and .edu. Also, the TLD for entities registered in some countries is an abbreviation of the country name and is called a country code. Examples include .uk for the United Kingdom, .ru for Russia, and .fr for France. When country codes are used, a second-level hierarchy for the TLD is often introduced to identify the type of the registered entity. For example, the .co.uk TLD identifies commercial enterprises in the United Kingdom.

Apex domain

The apex domain name includes and expands on the top-level domain. For domain names that include a country code, the apex domain includes the code and the labels, if any, that identify the type of the registered entity. The apex domain does not include subdomains (see the following paragraph). In www.example.com, the name of the apex domain is example.com. In www.example.co.uk, the name of the apex domain is example.co.uk. Other names that are often used instead of apex include base, bare, root, root apex, or zone apex.


Subdomain names precede the apex domain name and are separated from it and from each other by a period. The most common subdomain name is www, but any name is possible. Also, subdomain names can have multiple levels. For example, in jake.dog.animals.example.com, the subdomains are jake, dog, and animals in that order.


A fully qualified domain name (FQDN) is the complete DNS name for a computer, website, or other resource connected to a network or to the internet. For example aws.amazon.com is the FQDN for Amazon Web Services. An FQDN includes all domains up to the top-level domain. For example, [subdomain1].[subdomain2]...[subdomainn].[apex domain].[top–level domain] represents the general format of an FQDN.


A domain name that is not fully qualified is called a partially qualified domain name (PQDN) and is ambiguous. A name such as [subdomain1.subdomain2.] is a PQDN because the root domain cannot be determined.


The right to use a domain name is delegated by domain name registrars. Registrars are typically accredited by the Internet Corporation for Assigned Names and Numbers (ICANN). In addition, other organizations called registries maintain the TLD databases. When you request a domain name, the registrar sends your information to the appropriate TLD registry. The registry assigns a domain name, updates the TLD database, and publishes your information to WHOIS. Typically, domain names must be purchased.

End-Entity Certificate

An end-entity certificate identifies a resource, such as a server, instance, container or device. Unlike CA certificates, end-entity certificates cannot be used to issue certificates. Other common terms for end-entity certificate are "client" or "subscriber" certificate. We use the term end-entity certificate in the ACM Private CA documentation.

Private CA

Creates (issues) certificates for use within a private network or within an organization (that is, not the public internet).

Private Certificate

ACM Private CA certificates are private SSL/TLS certificates that you can use within your organization. Use them to identify resources such as clients, servers, applications, services, devices, and users. When establishing a secure encrypted communications channel, each resource uses a certificate like the following as well as cryptographic techniques to prove its identity to another resource. Internal API endpoints, web servers, VPN users, IoT devices, and many other applications use private certificates to establish encrypted communication channels that are necessary for their secure operation. By default, private certificates are not publicly trusted. An internal administrator must explicitly configure applications to trust private certificates and distribute the certificates.

Certificate: Data: Version: 3 (0x2) Serial Number: e8:cb:d2:be:db:12:23:29:f9:77:06:bc:fe:c9:90:f8 Signature Algorithm: sha256WithRSAEncryption Issuer: C=US, ST=WA, L=Seattle, O=Example Company CA, OU=Corporate, CN=www.example.com Validity Not Before: Feb 26 18:39:57 2018 GMT Not After : Feb 26 19:39:57 2019 GMT Subject: C=US, ST=Washington, L=Seattle, O=Example Company, OU=Sales, CN=www.example.com/emailAddress=sales@example.com Subject Public Key Info: Public Key Algorithm: rsaEncryption Public-Key: (2048 bit) Modulus: 00...c7 Exponent: 65537 (0x10001) X509v3 extensions: X509v3 Basic Constraints: CA:FALSE X509v3 Authority Key Identifier: keyid:AA:6E:C1:8A:EC:2F:8F:21:BC:BE:80:3D:C5:65:93:79:99:E7:71:65 X509v3 Subject Key Identifier: C6:6B:3C:6F:0A:49:9E:CC:4B:80:B2:8A:AB:81:22:AB:89:A8:DA:19 X509v3 Key Usage: critical Digital Signature, Key Encipherment X509v3 Extended Key Usage: TLS Web Server Authentication, TLS Web Client Authentication X509v3 CRL Distribution Points: Full Name: URI:http://NA/crl/12345678-1234-1234-1234-123456789012.crl Signature Algorithm: sha256WithRSAEncryption 58:32:...:53

Public Key Infrastructure (PKI)

A public key infrastructure (PKI) is a comprehensive system that enables the creation, issuance, management, distribution, use, storage, and revocation of digital certificates. A PKI consists of people, hardware, software, policies, documents, and procedures. In ACM Private CA, the purpose of a PKI is to manage private certificates. ACM Private CA implements the following:

Public Key Certificates

Your private CA issues certificates to affirm the identities of users, services, applications, computers, and other devices within your organization. For more information, see Private Certificate.

Certificate Repository

Your private CA stores all of the certificates it has issued. You can, of course, delete a certificate from the ACM Private CA repository.

Certificate Revocation

ACM Private CA supports certificate revocation. Certificates that can no longer be trusted must be revoked by the certificate authority. A certificate can be revoked before the end of its validity period for any number of reasons. Common reasons include key compromise, end of operations, a change in affiliation, or the withdrawal of privilege. When you create a private CA, you specify whether you want the CA to support certificate revocation. If the CA supports revocation, you can request an audit report to review certificates that you have revoked.

Key Storage

Your CA private keys are locked away securely in a hardware security module (HSM) that is owned and managed by Amazon.

Root CA

A cryptographic building block and root of trust upon which certificates can be issued. It comprises a private key for signing (issuing) certificates and a root certificate that identifies the root CA and binds the private key to the name of the CA. The root certificate is distributed to the trust stores of each entity in an environment. Administrators construct trust stores to include only the CAs that they trust. Administrators update or build the trust stores into the operating systems, instances, and host machine images of entities in their environment. When resources attempt to connect with one another, they check the certificates that each entity presents. A client checks the certificates for validity and whether a chain exists from the certificate to a root certificate installed in the trust store. If those conditions are met, a “handshake" is accomplished between the resources. This handshake cryptographically proves the identity of each entity to the other and creates an encrypted communication channel (TLS/SSL) between them.

Root Certificate

A certificate authority (CA) typically exists within a hierarchical structure that contains multiple other CAs with clearly defined parent–child relationships between them. Child or subordinate CAs are certified by their parent CAs, creating a certificate chain. The CA at the top of the hierarchy is referred to as the root CA, and its certificate is called the root certificate. This certificate is typically self-signed.

Secure Sockets Layer (SSL)

Secure Sockets Layer (SSL) and Transport Layer Security (TLS) are cryptographic protocols that provide communication security over a computer network. TLS is the successor of SSL. They both use X.509 certificates to authenticate the server. Both protocols negotiate a symmetric key to encrypt data that flows between the client and server.

Secure HTTPS

HTTPS stands for HTTP over SSL/TLS, a secure form of HTTP that is supported by all major browsers and servers. All HTTP requests and responses are encrypted before being sent across a network. HTTPS combines the HTTP protocol with symmetric, asymmetric, and X.509 certificate-based cryptographic techniques. HTTPS works by inserting a cryptographic security layer below the HTTP application layer and above the TCP transport layer in the Open Systems Interconnection (OSI) model. The security layer uses the Secure Sockets Layer (SSL) protocol or the Transport Layer Security (TLS) protocol.

Self-signed Certificates

A certificate signed by the issuer instead of a higher CA. Unlike certificates issued from a secure root maintained by a CA, self-signed certificates act as their own root, and as a result they have significant limitations: They can be used to provide on the wire encryption but not to verify identity, and they cannot be revoked. They are unacceptable from a security perspective. But organizations use them nonetheless because they are easy to generate, require no expertise or infrastructure, and many applications accept them. There are no controls in place for issuing self-signed certificates. Organizations that use them incur greater risk of outages caused by certificate expirations because they have no way to track expiration dates.

SSL Server Certificates

HTTPS transactions require server certificates to authenticate a server. A server certificate is an X.509 v3 data structure that binds the public key in the certificate to the subject of the certificate. An SSL/TLS certificate is signed by a certificate authority (CA). It contains the name of the server, the validity period, the public key, the signature algorithm, and more.

Transport Layer Security (TLS)

See Secure Sockets Layer (SSL).


In order for a web browser to trust the identity of a website, the browser must be able to verify the website's certificate. Browsers, however, trust only a small number of certificates known as CA root certificates. A trusted third party, known as a certificate authority (CA), validates the identity of the website and issues a signed digital certificate to the website's operator. The browser can then check the digital signature to validate the identity of the website. If validation is successful, the browser displays a lock icon in the address bar.

X.500 Distinguished Name

X.500 distinguished names (DN) are used to identify users, computers, applications, services, servers, and other devices in the X.509 public key certificates that ACM Private CA creates. This includes private certificates and private CA certificates. Common fields include the following:

  • organizationName (O) – Name of the organization that issued or is the subject of a certificate

  • organizationUnit (OU) – Department or division within an organization

  • country (C) – Two letter country code

  • stateName (S) – Name of a state or province such as Kansas

  • localityName (L) – Locality name such as Seattle

  • commonName (CN) – Common name of the certificate subject or issuer