[en] Using the Keck Planet Imager and Characterizer, we obtained high-resolution (R ∼ 35,000) K-band spectra of the four planets orbiting HR 8799. We clearly detected H₂O and CO in the atmospheres of HR 8799 c, d, and e, and tentatively detected a combination of CO and H₂O in b. These are the most challenging directly imaged exoplanets that have been observed at high spectral resolution to date when considering both their angular separations and flux ratios. We developed a forward-modeling framework that allows us to jointly fit the spectra of the planets and the diffracted starlight simultaneously in a likelihood-based approach and obtained posterior probabilities on their effective temperatures, surface gravities, radial velocities, and spins. We measured $v\sin <!--\; ???\; -->(i)$ values of ${10.1}_{-<!--\; ???\; -->2.7}^{+2.8}\mathrm{k}\mathrm{m}{\mathrm{s}}^{-<!--\; ???\; -->1}$ for HR 8799 d and ${15.0}_{-<!--\; ???\; -->2.6}^{+2.3}\mathrm{k}\mathrm{m}{\mathrm{s}}^{-<!--\; ???\; -->1}$ for HR 8799 e, and placed an upper limit of <14 km s⁻¹ of HR 8799 c. Under two different assumptions of their obliquities, we found tentative evidence that rotation velocity is anticorrelated with companion mass, which could indicate that magnetic braking with a circumplanetary disk at early times is less efficient at spinning down lower-mass planets.