---
title: "LMS Keys - FAQs"
slug: "lms-keys-faqs"
updated: 2025-03-12T07:54:57Z
published: 2025-03-12T07:54:57Z
---

> ## Documentation Index
> Fetch the complete documentation index at: https://support.fortanix.com/llms.txt
> Use this file to discover all available pages before exploring further.

# LMS Keys - FAQs

## 1.0 Frequently Asked Questions

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### 

****What are Fortanix's limitations on LMS Key tree size?****

Fortanix LMS implementation is a two-level Hierarchical Signature System (HSS) whose heights are a list of heights [`h₁`, `h₂`] respectively. Where `h₁` is the height of the top-level tree and `h₂` is the height of the secondary-level tree. The sum of the `h₁` tree height and `h₂` tree height must not exceed `25`. For example, two trees with heights `h₁ + h₂ <= 25`. Where `h₁` and `h₂` are multiple of `5`.

****How many signatures can an LMS key perform for the lifetime of the key?****

The "key capacity" is the number of signatures that an LMS key can perform for its lifetime. It is equal to `2 ^ (h₁ + h₂)`. For example, a (5,5) key or tree can produce 1024 signatures.

****What does an LMS signature look like? How would it be verified by an external library?****

*See*[*RFC8554 - Section 6.2*](https://datatracker.ietf.org/doc/html/rfc8554#section-6.2)*and*[*RFC8708 - Section 2.2*](https://www.rfc-editor.org/rfc/rfc8708.html#name-leighton-micali-signature-l).

> [!NOTE]
> NOTE
> 
> Since there are two trees (`h₁` and `h₂`), there is only one signed public key. Fortanix also includes the public key. One LMS signature that is needed to define HSS on the next layer is serialized into `1132 + 32 × h` bytes.

****Is there a way to check the number of signing for LMS keys?****

Users need to keep track of the number of signatures that are produced.

****What RSC are we using for LMS-based certificates?****

Fortanix does not format LMS keys in certificates.

****Can the Fortanix PKCS#11 library handle verification for signing for external apps?****

PKCS#11 will verify any valid signature within the available [PKCS#11 supported mechanisms](/v1/docs/pkcs11-supported-functions-and-mechanisms#mechanisms-supported) as long as the input signature is in a format that PKCS#11 understands. LMS is not supported in PKCS#11.

****What are Fortanix’s stance and best practices for CSR generations?****

Some best practices are:

- Add the CA key inside Fortanix DSM and adequately verify the CSR for authentication before signing.

#### Do the number of signings start over with an LMS key if the key is rotated?

Yes. It starts over.

## Related

- [Add/Update Cryptographic Policy](/addupdate-cryptographic-policy.md)
- [What is cluster attestation and why should I use it? (on-prem only)](/what-is-cluster-attestation-and-why-should-i-use-it-on-prem-only.md)
- [FAQs - Fortanix Data Security Manager UI](/faqs-fortanix-data-security-manager-ui.md)